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9
1924
NEW YORK ACADEMY OF SCIENCES
SCIENTIFIC SURVEY
OF
Porto Rico and the Virgin Islands
VOLUME II—Part 2
The Geology of the Humacao District, Porto Rico—
Charles K Fetike
NEW YORK:
Published by the Academy
1924
f
NEW YORK ACADEMY OF SCIENCES
SCIENTIFIC SURVEY
OF
Porto Rico and the Virgin Islands
VOLUME II—Part 2
The Geology of the Humacao District, Porto Rico-
Charles R. Fettke
NEW YORK:
Published by thk Academy
1924
THE GEOLOGY OF THE HUMACAO DISTRICT,
PORTO RICO
By Charles R. Fettke
CONTENTS
Page
Introduction
118
Location
of the
district
118
Field work and acknowledgments
ll{)
Previous work
120
Physiograpliy
120
Upland area
121
The upper peneplain
125
Monadnocks
127
The lower peneplain
127
Caguas basin
128
Coastal area
129
Promontories
130
Playas
130
Drainage
133
General geology
134
Formations present
134
Older series
.'
137
Tuffs and
volcanic
breccias
137
Andesites
142
Felsite
147
Limestones
148
Collores
limestone
149
Tuffaceous limestones and calcareous
tuffs
150
Shales
151
Sandstones
152
Intrusives in the older
series
152
Intrusive
sheets and
sills
152
Stocks,
bosses, and
batholiths
153
Juncos gabbro
153
San Lorenzo
quartz
diorite
153
Yabucoa
granite
159
Patillas quartz monzonite
161
Dikes
163
Siliceous dikes
1^3
Granite and granite pegmatite dikes
164
Aplite
dikes
165
Granite and rhyolite porphyry dikes
165
Rhyolite
167
Dikes of intermediate and basic composition
167
(117)
118
SCIENTIFIC SURVEY OF PORTO RICO
Page
Latite and monzonite dikes
167
Dacite and quartz
diorite
dilves
168
Andesite and
diorite
dilves
168
Diabase dikes
170
Contact metamorphism
170
Hornblende
schists and
associated rocks
170
Metamorphosed limestone
176
Quartz diorite gneiss
178
Folding
179
Faulting
179
Alluvial deposits
179
Historical geology summary
181
Economic geology
184
Magnetite
deposits
184
Road metal
194
Limestone
195
Brick
clay
195
Kaolin
,
195
Thermal
springs
195
Bibliography
197
INTEODUCTIOlsr
Location of the District
The
district described in
this report covers an area of approximately
400 square miles in the extreme southeastern portion of Porto Eico.
It
has been
arbitrarily bounded on
the north by
the
parallel
of 18°
15'
north latitude and on the west by the meridian of 65°
5'' west longitude.
The Caribbean Sea
lies
to the south and
east.
The
district has been
named
after the town
of Humacao,
the
capital
of
the Department
of
Humacao^
the southern half
of which
is included within
the
area
de-
scribed .
Important towns along the coast, starting at the southwestern end, are
Arroyo, Patillas, Maunabo, Yabucoa, Humacao, and Naguabo.
Interior
towns
in
the northern portion
of
the
area
are
Caguas, San
Lorenzo,
Juncos, and Las Piedras.
The interior portion of the district is a rugged
mountainous area in which only small villages are located.
Caguas, in
the
northwestern
corner,
is
connected
with
San
Juan,
the
capital
of
Porto
Rico, by an
electric and steam
railroad.
In
1916
constru^ction
work -was under way
to similarly connect Humacao with Caguas.
The
only other railroad lines in the district have been built along the playas
and interior valleys to furnish transportation
facilities from the sugar-
cane
fields to the centrals or sugar mills.
These are of only light con-
FETTKE, GEOLOGY OF THE HUMACAO DISTRICT
119
structioii and are not connected with one another.
Good roads connect
the coastal towns and those of the northern portion of the
district, hut
the interior
is only accessible by native trails which have to be traversed
on horseback or afoot.
Field Work and Acknowledgments
The geological survey of the Humacao
district was made during the
summer
of
1916
under
the
auspices
of
the Kew
York
xVcademy
of
Sciences and
the
Insular Government
of
Porto
Eico.
Field work on
the area was commenced on Jul}^ 8 and completed on Septeml)er
3.
A
map
of
the Island
of Porto
Kico,
compiled by
the Bureau
of
Public
\¥orks of the Porto Eican Department of the Interior under the direc-
tion of John A. Wilson, Commissioner
of the
Interior, and dated May
13, 1912, upon which the locations of the principal streams, roads,
rail-
roads, towns, and villages are shown, was available as a base.
The charts
of the United States Coast and Geodetic Survey, which accurately show
the configuration of the coast^ were also used.
In the southwestern corner
of
the
district,
the
Arroyo-Patillas
sheet
of
the Porto Rico
Irrigation
Service, drawn to a scale of
, covers a small area in great detail.
12,000'
^
The various towns along the coast and in the northern portion of the
district were used
as headquarters
as
the work
progressed.
Pace and
compass traverses were made along
all of the roads and also along the
greater portion of the
coast,
as well as along many of the
trails which
follow
the
principal
valleys
into
the
interior.
Two
north and
south
traverses were run
across the eastern extension
of the
Sierra de Cayey
in the western part of the district.
On account of the limited time avail-
able and the
fact
that much
of
the work was done during
the
rainy
season, heavy
tropical showers being encountered nearly
every
day, no
attempt was made to sketch the topography.
Seven hundred ninety-six
rock
specimens were
collected during
the
course
of
the
investigation,
from 394 of which thin sections were later cut and examined under the
microscope.
All the data gathered have been assembled on a map drawn
1
to a scale of
.
62,500
The
office and petrographic work in connection
with the preparation
of
this report has been
carried on
in
the
geological and mineraloglcal
laboratories of the Carnegie Institute of Technology at Pittsburgh, Penn-
sylvania.
Its completion has been unduly delayed,
first, by the entrance
of the United
States
into the World War which
attracted the
writer's
120
SCIENTIFIC SURVEY OF PORTO RICO
attention to other imperative calls, and later, by an investigation started
at an
earlier
date
for
the
Pennsylvania Bureau
of
Topographic
and
Geological Surve}^, for whose completion funds became available.
The present investigation was undertaken
at the
suggestion
of Pro-
fessor
Charles
P.
Berkey,
of Columbia
University, who
furnished
the
writer with much valuable information derived from his own experience
in making a reconnaissance survey of the Avhole island.
Dr. E. T. Hodge,
who had made a geological survey of the Coama-Guayama district, which
adjoins the Humacao area on the west, during the previous summer, also
gave helpful advice.
The writer
is
also indebted
to Colonel George E.
Shanton, Chief of the Insular
Police.
The
hearty co-operation
of the
insular police of the various towns of the
district, obtained through his
letter of introduction, did much toward facilitating the work.
Eduardo
Martinez, of Comerio Falls, acted as assistant and interpreter during the
entire field season.
His faithful and intelligent service was an important
factor in enabling the field work to be brought to a successful completion
within the limited time available.
Previous Work
Berkey's
(1915)
report on the general geology
of Porto Eico,
based
upon a reconnaissance survey made by him and C. N. Fenner in 1914,
was found an invaluable aid in carrying on detailed work in the Humacao
area, and frequent reference to it was made in the
field.
Other articles
dealing with the geology of Porto Eico, published
prior
to
this, make
only casual reference
to the geology of the southeastern portion
of the
island and give very little information of a specific nature.
A list of such
articles is included in the bibliography attached to Berkey's paper.
Since 1916, detailed reports on the geology of the San Juan (Semmes,
1919), Coamo-Guayama
(Hodge,
1920), Ponce
(Mitchell,
1922), and
Lares
(Hubbard, 1923)
Districts and the Physiography of Porto Eico
(Lobeck,
1922)
have
been
published
by
the Kew York Academy
of
Sciences.
The information contained in these has been very helpful in
interpreting some of the data obtained on the geology of the Humacao
district.
PHYSIOGEAPHY
The Humacao
district, taken
as
a whole, has
a mountainous
aspect.
It is a region of rugged relief.
A considerable portion of the west cen-
tral part
is occupied by the eastern end
of the
Sierra de Cayey, some
of wdiose peaks reach an elevation
of 3000
feet.
To the north
lie the
FHTTJ\h\ (U-:iHJ)<n-
(iF THE IILWLICAO IHHTMICT
ni
Lii(|uillo
moiiiitairis whidi
rise
to
still greater
elevations.
Tlieir south-
em
Bloper- an*
iiu-liHlcd
in
part within
the buumlaries of the district.
The Tertiarv
(-oastal
plain, which
is proiiiinetitly developed along the
northern
an<l
southern coasts of
the
iskiHl.
is
iibscut
in
tiie llomaeao
(iistrict.
till!
coast
line
l»ein,i:'
charaeterizerl
inrteacl
bv
bold
headlands
sef)a rated by broad
alluvial
rrllcd
valleys.
In
llio
northwestern part
oC
the
(lisiTiet
tin: Caguas
valley,
-a broad
interior lowland, forms
a. rather
hi r i k
i ug
ttvpcigraph ic
feature.
I'i'LAXn
iVl.'KA
The
uplauil
area, whieli eompri.ses
tiie
maJDi' portion
oC the Huoiaeao
district,
is
nriderluin
largelv bv
a comiilex mass of folded volcanic
tiUTs,
Fi«.
].- Mride
between
FntUhu
nnd
Dvl Mnimw
drahmge
mj«temis
breccias,
an<l
lava
flows intruded
lyv
large
bodi(»s
of quartz
dioritc and
granite.
It has reached a matun; stap:e of erosion and lienee exliihits an
extremeh- rugged aspect.
The Inmud tropical climate
Inis l>rought about
the developmeut of a tine textured topograpliy.
Weathering nearly cvery-
wduu-e has proceeded
to consid'erable depths so
tliat outcrops of
tfie bed^
rock fomiatituis are
scarce even
in
the extremely uiouotaiuoug portiims
or
tlH^» disti'ittt. except where the streams have eui througli the soil
mantle,
rroruineut
talus
slopes,
so cluiracferistie
of mouutainoiis regions
wliere
nujre
rig(u'ous diuiates
i)revail, are
praetieally
absent.
Figure
1 shows
the appearaiu-e of the ridge in the Sierra de Cayey wdd,cli ttmns tfie divide
between
tfie PatiUas and
Del
Flspiru^
draiu,ag<'S.
Only
in
a few places
along
it do any outcrops of
tfie underlying fornudi(»ns appear.
122
,s('!EXTIFJ€ 8('in'MY OF PORTO HH'O
(liemical
dei-omposition Mather
tliiui
inccJiaiiicHl
disinro^ixmiion,
has
lioeii
tlio
prevailing
process
of
weathering.
Rofk<
likf>
quiirtz
diorite
frtM|ueiitly sbow etelied surfaces, where the soil has been removed, .^iinihir
to
those
oi"«li]iari;ly
associated
with
the outcrops
of more
^ohilde
rocks
such
as limestone.
Outcrops of quartz
diorite
affected
in
this
luanni'jr
appear in
tig'unis 2,
3, and
4.
As may be seen in
fiofiire
2. the
tiixuriaiit
tropical Yegctation^ probably plays au imporliiut role
in,
llic devclupjuent
(d'
thcs<'
wcatlicred
surfaces
widcli
are
so
unusual
for
quartz
cJioritc.
\"cry tenaeiou,s
(daytjy
soils arc formi'd.
Hteep
slopes,
ranginii' from
;>(>
to do
degrees with
cuchillo
or
knife^ed,ge
divides,
are
cbaraetcristic
of
—Oiiterop
of
qtnirtz
dh
tli(» mature topography
de\-eloped,
aiid
the
striking
feature
is
tluit the
soil clings to tliem ajul frequently tliey are under cultivation.
Figure
5.
taken
near
tlie
headwatx:»rs
of
the Turaho
lliver
wdiere
the
l)edrock
is
largely an
aiidesitie
tuff,
is typical
<jf
this kind of topography.
Native
huts with
little ])atcbes of cultivated ground
artvund
them can be
seen
at numerous places ahuig
tlie steep
hillsides.
Figure
(i shows a
trail
in
the quartz diorite area
in
the region of the lugenio valley, which
starter!,
origi,iially on
tlie surface of the
soil majitlc overlying
tlie quartz diorite
hn,t
lias
j„iow been worn down
n,early
t«
liedrock
f>y
tlie surface
run-o,tf
wldcli flows along the depression started by
tlie traffic over
it.
The steep
walls on
either side are very characteristic of the manner
in which the
residual soils of Forto
tiico stand
uji,
Bcrkcy
(191,5,
p.
:ii)
b(d,ieves
tliat the three
,factors
tluit are
largely
FETTKK, CEiUJMlY Of THE HUMAiWd
IUf<TUI("r
|23
Fluted
tpiftftz
rcHpoiisiljIe
for
tlio remarkaljle stubiJity of the
i^oil
inniitlo
iire
tlie
eliiig--
iiig
charaeter
of
miicli
of
ihe
vegetation
whieli
teiid.<
tu
bind
the
soil
together,
the
small range
oJ'
tein])eri;itnre
variation
whiefi
rediiee.i
ine-
ehauieal (Ikiutegration or disruption tondeneies
t'> a
niiiiiininn, and
ifie
low
(X)nt<>iit of hwrX or refractory materials,
snoli
as
(jnartz, in
tlie
rock;^
^^fc'lfttf&^Rfcin
Fi<;. A.^^^-DetaU of
(lutiHti hHuwh
in figure 3
SCJEXTIFIV ISUEVEY OF
IHilfTO RH'O
wliuHC
<l(•^;^rvK•tu>ll
hrts furni^ihwl the
soi'l^.
Lt^hei-k
(1!»22,
]>.
/.OS)
ihi,|iks
that
aiujtlier
factor
is
ilie 9\\mi
cliiratioii
ix\\{\
1i>rromi}il
clinriu-tcr of
tJu'
riiiiifall
whifh
n-sults
in
ati
execcHliiigiy
hcavv and
rapid
nui-ojr
ovi;r
the surface of
t'he
iiiiperviuiis soil.
Ill
iny
opiidoji,
tlic
ehariieter of the
weatlu'rijig
process
iiiuler condi-
tions
such
MS
prevail
in^
an
extreiiiel_Y
Jiunud
ti'opieal
climate,
whieli
Unii^-.
:|iH.i)i
n
ciKii!ir:!l
i!cMUnpositio!i
r;il!h-i-
Shan
a
mt tv
ni-'-hiinicni
diHn!c;M';nu^j:
.:i
ll,c
*xp.is>.|
vuik>
iUid.
u-
a
i-csuh.
i>n>din-rv
a
^crv
FhTTKI'I, (IBOLOiiY Of THE IICMAVAO UJiSTRli'l'
ia5
the behavior noted.
Thi.>
t^oil k not >iihjet*t to the action
o1*
frost which
i.« mivh a potent
factor
in bringing
rtbout shmiping
in
uokltn-
climates.
Its impervious imtnre vamei^ mueli of the rain whieli falk upon
it to rmi
off rea<Hly
over lU
mrinv.e.
As
»
result,
true spriugB are
of compara^
lively rare oreurrenee in
tlie distriet.
The abnenee of quartz from many
ol tlie rocksarrulonbtetlly is also an important: factor, although the author
ohserve«l many stoop slopes under eullivatinn and saw soil-eovered knife-
ed^ge ridges in the quartz diorite
ari'H.
l)iitero])S
oi: the
l>edrock forina^
irm
Uidiic, a
imrtUtii
nf Jahiivun Hfunr
tiojis along the divides, howev(»r, are of much more common occurKince
in the region underlain by this rock.
Tlic drainage system develfjped upoJi
tlie upland area
is of
tlie typical
dondritio
type.
In
general
it may
l)e
said
that
the
character
of
the
under]yin4' formations and tbcir structure
bias had very
little influence
upon^ the directions
iji which
tlie streams lurw
flow.
Much of the area
underlain by
cpjartz
diorite
has
been
^ worn
to a somewhat
lower
level
tlian those portions of the district in whieli
in,du,rated volcanic
tuffs and
breccias
and
aJidesites
constitute
the
bedrock.
On
tlie
other
liand,
Panduras Kidge.
sliowii
in
figure
1',
wliic-h
etuistitutcs one
of
the most
imposing physiographic
features along
the
soutliern
coast,
is
underlain
by (pnirtz diofite.
TITK
i:ri'KR FEX-EPLAIX
Over mucb
of the western
portion
of the u|)land area
the
c-m-u
crest
line, formed by the genera!
aceordaiu-e
of
tlie snmmit
levels
of the
in-
numerable
rido-es,
is
a
particularly
striking
feature.
Bucks,
including
SCfEX'llFie HVh'VEY OF POUTO RH'O
jiuiiii-ulfcd
ujii:tiiiJA:
luilri
iiiui
bi:t:i-cja>,
aJidu^ik-c.
ijLUjrLz
diuriu;;-,
aihi
irKtii;<oiiit:fs are
beveled
alike
at
this
level.
Apparently
this
region was
at one time
redneed
to
a,, peneplain whieb.
has
since
Ixseii
eltn'ated and
niittnrel.T disseeted.
The area over whieh this ohl erosion
surfax-e
(uui
be
recognized comprises that portion of
tlie upland of the lluimieao
clistriet
f'xte:iidinrt- I'rom its 'western border to a line east of the Del Kspino Eiv^r,
rnnning a|»|>roximate]y from San Lorenzo
to Yabueoa,
On the north
it
is bounded
h}-
flie (Jaguas lowhuid and on the south
it extends nearly
to
Pic.
0.
ilattirelv
Mssected upper peneplain
in t'Mmty
of Mulns
barrio
FF.TTKE, ah'OlJ'HJY OF THE HVMJflAO IHHTMICT
12T
llif eoast.
The
cvfrn crest line prodnecMl hv thf erosion
rt'Jiiiirtiits of
thif>
old
])»;'iiepJaiii
t-au
he
Heoii in
fiji,iir(;' S
irbicli was taken
liM)kin<x eastward
across the headwaters of the
Del Espino River and
in
figure
!>, taken
in
the
vieinitv
of Hulas
barrio.
In
tiie
eentral portion
erf
the
area,
this
pejiephiiu level lias an eh:'vatiou of approixiniately 2(100 feet.
, Xo distinet
traees of
it
\xer('
reeognJzeil along
the
,soiith.ern
Hlo|)es
oC
tlie
Lucpiillo
niountairis within the limits of \h(^ Hnmaeao
rlistrict.
The peneplain
(ieserihed
al)Ove
is
iiiHloiibtedly
the one
wliieh Berkey
(1015,
]).
50)
recognlzetl
in
his reconnaissance study
<d; Porto
J»ieo
in
15)14.
Lobeek
(1922,
pp.
:J12-:;I20)
has
referred
to
it
as
the
first
or
higher penej)hd]i to distingiiisli
it fi'om
a second or lower one
wliieli was
develo|)ed over
|)orrions
of the
island and which
is
also represented
in
the Hrtniapao di-trift.
MOXADXOCKS
Along the divide between
tlie Del Espjiio ami
l.a llata
Bivto-s
on^ the
north and
the streams that tk)W southward
into the
C'arilthean
Sea,
pco'-
tions of the Sierra de
(^'aycy stand ont as nionadiujeks ahovc
tlie level of
the
n])per
peneplain.
Some
ef
tlie
higher peaks
rise
nearly
IdlK)
feet
above the peneplain
level.
The
Luqiiillo
niooiitains, whidi
cxtenil
irdn
the northeastern
jiortion
(,»f the
district,
have
a
sinnhir
relationship
to
this
<)ld erosion snrlaee.
xriE LOWKR
i'i':xi:r;L.iLv
A
Itnver
erosion
siirfaec
also
njarked
by
concordant summit
levels
eeui'S between
the (fuavanes and
lliiniaeao drainaee
areas.
The
rela-
138
saiKNTiFJC suHrnr of p(H{TO iuvo
tionsliip of
tliis lower piMieplaiii
tx>
the upper one can
be seen
in
figure
10,
wliieli
is:
a view taken about 4
uiiles northwest
of Yaliueoa
lookiiiij
uortliwest across tlie lower erosiou s^urface toward tlie upper which
foririf^
the crest of
tlie esear])meut
neeii
in
tlie
haekg-roaiHi
Tliis
Io\Yer
}>enc-
plain
lies alwnt lOOO feet below the upper.
('A<h;as basix
An
extensive lowland,
jrraetieally snrrou ruled
on
three
sidet?
by
h\<ih
hills whose crests rise
to
tlie level of
tlie upper peneplain, has been
d^'-
veloped by
(\rosioji
in the iiortliwestern
portiioi of the
I'Tuniaeao
distri<'t.
Its avera^^e elevation above sea-level
is aliout 550
feet.
On, the
iiortli
it
extends beyond
the limits of the
district.
On
the west and
sontli
it
is
Ijounded by an
uplan<l
w'hose summit represents
t.h,e
level
of
the upper
peneplain.
On
the
S(uitheast
a
low divide
sejjarates
it
from
tlu' lower
peneplain
area
previously
described.
Figure
11
is
a
pliotograph
tak(n,i
in the western
portion
of the lowland
along the C'aguas-Aguas Buemis
road.
The upland area
oji the
scnith can
be seen
ii> the background.
The Cagnas basin
is drained by
the Turabo, De Espiuo, and Ciurabo
Rivers
wliicli
mute
to form
the Grande
de
Loiza.
The
latter
stream
leaves the basin at its northwest corner where
ir has cut a valley into thn
upland
area.
The (jrande de Loiza and
its
tributaries have developed
l)road
flood
piaijis ahmg
tlieir courses which
lie from ?M)
to
:iOO
feet
below the level of the lo^vland.
Lol)ei'k (H)3S, p. Ml 8)
belit-ves
tlui4. the Caguas lowland was developed
at the same time as
tlie lower peneplain.
It stamls at a scmiewluit lower
VF.TTKH, (JHOLO'fjy OF THE HUMAVAO Dh^TRICT
*#. .^,
|^^:^'J
^;i^i(
FUi.
12.
-irwr.
IM
f/ic nrinihi «f Qiiebnula
llomla
fni.tit
levi'],
liowever,
than
the
surface
of
tlii.s
pwiepkiii
rt.<
<'U>vol()|)e<l
in
the
eastern
|)(irt;i<m of the islaml between, the Unayanen and Tlninaeno Jiivcrs,
Co.\STAL
AiiEA
The
coast
line
of
that
pintinn
of
I'urto
Jiu-o
inehnled
witliiii
the
limits
o! the
Ilmnacao
ilif^lriet
is one produced by
dnnvnj'iig
in
whicli
the
Hubinerg-ed
valleys have
l)een
filled
with
alhiviiini.
Wave
eronion
upon
ttie Jieadhinds and the
filling of the drowned
valleys,
partieulaTly
the latter, lutve to a eonsidi-ralde extent obliterated the mon- i)ronouneed
^'^-tr-1;''^-sS-
?. '
.
#**
"..
il l:.*-.H;%:i-
v.,s-il»rft:-
1;:
::;:.
SCtENTIFIV SUKV'EY OF PORTO RHjO
irrognliii'itie?i m
tJiat,
with
the
p<)S^^il)h'
oxec'ptioii
of
Eiisonada
llniidrt. Ihoro are no «-oo(l harbors.
liiOMONTOKfE.'^
At
mirnermiR
phn'e^
along
the
s;iiithi;'ast
coast
of
Port(»
Rico,
lH.«<llaiHls made
up
of the
ohicr
tormatioiiH of the
iiphind
area or
i«i,M!Ooii!s
rocks
intrij.^ive
in them
extend
to
tlie
s^ea
wliere
they
ternunate
in
wave-eut
cliffs.
Amongst
tlie most
imiiressive
of
tliese
is
!Pandn,ras Ridge between
^rannaho
and
Yalnieoa.
Other
•-
prominent ones oc(:nr siiutheast of
-f
I'atillas
and
southwest
of Man-
^
iml>o,
also
het-ween
Yabneoa
anxl
i
Candeh'ro
|)hiyas,
('andtdero and
-<
irunja<'ao playas. ajul soidheast of
'~
Xa,i;nabu.
J
In
ijhu-es
the
headlands
are
I
iiotclietl hy wlnit are |n-ot»abIy rem-
:?,
luuit-
of
marine
terraces.
Two
I
such
terraces
in
tlic
vicinity
of
'^
Qnehra<la
llojuJa Roint are shown
:i
in
figur<; 12.
Tlie npper of these
i
is
approximately
280
feet
ahoA-e
'"
sea-Un-el,
whih? the htwer one
oc-
enrs
at an
elevation
of
1CK)
feet.
Fignre
13 shows anotlier of these
terraces
in^
the
vidinity
of
Na-
guaho
Rhiya,
which
is
of
mueh^
more
reci-Jit
origin
and
stands
only
a
short
dJstanct;
alcove
sea-
level.
The
hroatl alluvial plains slop-
ing
gently
toward
tlic
sea
that
ceeur along
the lower
courses
of
all
the major streams
al(U)g
the
southeastern
coast
are
locally
known
as
playas.
For the most
FKTTKi:, CEOLOdY OF THE MUMACAO DIHTlllCT
pnrt,
tliev
ri-proKoiit est4iaries
fofinecl bv drowiiiiig
tliiit
luivo been
filled
with
alliiviuiu brought down by tbe
streaius.
Inland
they often
g-ratle
into
typifjil
J1<hhI
plain?*
dev<dopod
by
the
widening
out
of
tlie
river
valli-y:^.
Fio-nro M
is
a panorama
view
of
tlie
I'atillas
pbiya,
while
fe^*t
"m^ 'm^:^&
FffENTIFlP PURVEY OF POKT(f RfPO
llotida and
Yabueoa.
Pointn.
In
thn
former,
the
ii|>{ier
|)ene[>laiii
level
is readily
recjogiiizaljle along
the
crests of
ilie
liills which, bor-
<ler the Irroad
valh:\v.
One of the
oiitstHiiding topographic
features
along
the
southern
c:oast
is
the
abruptness with
which the moun-
tains
rise
to
this
h'vel
In
the
ease of the Yalujcoa
phiya, there
is
an
al)sejice
of
a
pron,oimced
I'ave-cut
I'ljll ah.Hiw- the shore line.
This
is
cfiaraeteristie
of most
ol
th,e
phiyas.
The
sandy
beach
marking
the
playa
margin
is
usually
lK)r<lere<l
by
a
zone
of
eocoaniit
|)ahns.
l»ack
of
wdiieh
frequently ocmr
small
areas
of
salt marsh,
an,d
beyond
which
in
turn
lie
fertile
fields phmtcd
in
sugar cane.
Xot
all of the lylayas are of the
cstuarine
type.
Along
tlie south
Coast some,
sucli as the one upon
whicli the town of Arroyo
is built
and
the one developed
in
the
vi-
(•inity
of
Viento Point shown,
in
figure
1(i, are marginal and more
in
the
nature
of
alluvial
fans
often
possessing
a
i^rououu
seaward
slope.
"\Vhik'>
in most cases
tlie
i)laYas
•jiri'
not
ternnnated
by
any
pro-
nount-eil
cliff
along
their
sea-
ward
margins,
one
striking
ex-
ception
was
noted.
Figure
17
sliows
a
cliff
,^5
feet high cut by
the waves into the nnconstilidated
playa deposits a]<mg the coast just
beyond
the
western
limits
of.
the
Hunnica«,>
disti'i('t.
The
uppei'
:>
feet of the alluvial dei)Osit, as nuiv
FETTKE, GEOLOGY OF THE HUMACAO DISTRICT
133
be seen in figure 18^ are extremely fossiliferoiis.
Abundant shells of many
recent species of gastropods^ pelecypods, as well as coral remains in every
way
identical
with
those
accumulating
along
the
present
beach,
are
present.
Indian pottery fragments
occur
in the lower portion
of
this
fossiliferous horizon, which are associated with the shells in such a man-
ner as to lead one to the conclusion that they were deposited contempo-
raneously.
The great variety of genera and species and the lateral extent
of the
shell bed seem to indicate that the deposit does not represent an
Indian shell heap, but rather that there has been along this portion of
the coast an uplift of from 35
to 30
feet since the region
first became
inhabitated by the early Indians.
Draixage
The
northwestern
one-third
of
the Humacao
district
is
drained
by
the Gurabo, Valenciano^ Del Espino, and Turabo
Kivers,
tributaries of
the Grande de Loiza which
enters the Atlantic Ocean along the north
coast east of San Juan.
Some of the headwater tributaries
of the La
Plata, which enters the Atlantic west of San Juan, also
rise within the
western limits of the district.
The rest of the area is drained by streams
flowing southward and eastward into the Caribbean Sea.
One of the outstanding features of the drainage
is the unsymmetrical
position
of the
divide between the streams which flow northward
into
the Atlantic and those that flow southward and southeast\yard
into the
Caribbean.
As an example, the Del Espino
Kiver, which
rises on the
northern slopes of the Sierra de Cayey, and its continuation, the Grande
de Loiza Eiver, have a total lengtli of 36 miles, while the Patillas Eiver
which
rises on the southern
slopes of the same range opposite
it has a
length of only 8 miles.
Berkey
(1915, pp. 39-41) and Mitchell
(1922,
pp. 266-269)
have traced a prominent fault
of
late Tertiary age along
the southern margin of the island extending from the vicinity of Juana
Diaz to a point 5 miles east of Point Melones on the west coast.
Accord-
ing
to
Mitchell,
the displacement
in the
vicinity
of Juana Diaz
is
at
least 900
feet and probably
considerably more.
The
presence
of
this
fault and
the unsymmetrical
arrangement
of
the
drainage,
as
well
as
the comparatively abrupt manner in which the
island
is terminated
at
both the east and west ends, has led Berkey to conclude that Porto Eico
is probably bounded roughly on three sides by faults
(the east, west and
south)
and
is a block
tilted gently
to
the
north.
Tbis would exphxin
the drainage phenomena noted.
Lobek (1922, pp. 304-305), on the other
hand, believes that the greater length of the streams flowing nortliward
is due to the fact that the precipitation
is much greater on that side of
X34
SCIENTIFIC SURVEY OF PORTO RICO
the island.
He attributes the phenomena along the south coast^ ascribed
by Berke}^ and Mitchell to indicate faulting, to be the result of the sag-
ging and
collapse
of cavern
roofs
in
the Tertiary
limestones.
If
the
unsymmetrical drainage
is due
entirely
to
differences
in
rainfall, how-
ever,
it
is
difficult
to
explain why the
lofty Luquillo mountain group,
with very short streams flowing northward to the
sea, should be located
in the extreme northeastern portion of the island where the
rainfall
is
heaviest.
The
character
of
the rocks that underlie
this area does not
appear to be essentially different from those of the rest of the island.
Stream terraces occur along most of the m-ajor valleys of the Humacao
district.
Particularly good ones may be seen along the Gurabo, Valen-
ciano, Del Espino, and Turabo Elvers.
In most instances these can be
explained by the
fact
that the
rivers have changed from aggrading
to
degrading streams within comparatively
recent times and they do not
necessaril}^ indicate that an uplift has taken place.
GENERAL GEOLOGY
The
interpretation
of
the bedrock geology
of
a large portion
of
the
Humacao
district
is difficult on account of the thick mantle
of residual
soil which covers most
of
it.
In the
lowlands, outcrops
are
extremely
scarce; while in the mountainous
areas, where they are more
plentiful,
they are frequently
so badly decomposed
as
to make
the determination
of the exact nature of the original rock almost impossible.
Pormational
boundary
lines
can,
therefore,
in many
instances
be
traced
only
ap-
proximately.
The best exposures
are found along stream beds
as shown
in
figure
19, along the
cliffs which the waves have cut back
into the headlands,
along road
cuts, and
occasionally
at
or near
the
crests
of
the
ridges.
Often residual boulders in the thick mantle of soil furnish the only clues
available
in regard
to
the character
of
the underlying rock
over large
areas.
The formations have been folded and invaded by numerous
in-
trusives,
but
strike and
dip measurements
can
rarely
be
obtained
on
account of the massive character of most of the rocks involved and the
scarcity of outcrops.
Formations Present
The oldest formations exposed in the Humacao district belong to what
Berkey has called the Older Series.
Berkey
(1915, p. 10)
considers the
whole series to be of Cretaceous age, using the term to include both the
Upper Cretaceous and Lower or Comanchean.^
Later workers in other
^ Op.
cit.,
p.
58.
FETTKI'I, UEOJAHJY OF
TH!-J JIUMAAJAO DIKl'MH'T
135
parLs
«tf
Portxt
Hico
{-oDfur with
him
pretty
well
in
tliis
correlatioji.^
Se-iiiHies
eonc'liuTos from
foratniiiifcTal
reiriains
foiiruJ
in
the upper por-
tion of the older series
in the San Juan distriet, northwest
iif
tlie Hnma-
i'-i
2 Bemmes,
I».
It.
<}»>oI<>gy
of
the San
.luan
district.
Scientific
Siirvfy
of Port© Mm
and the Virgin
Islands, Xew York Aciulemj of Science.
Vol.
I.
Part
1.
pp. T2-?S.
Hodge.
E.
T.
Geology
of
the Coatno-Guayamu
distiict.
Idem.,
I'ol.
I,
Part
i,
pp.
Mltcliell,
ih
.T.
Geology
of
the Ponce
dlslrlct.
Idem.,
Yol.
I,
I'art
3,
pp.
SSi-iSf.-
Hiibbsircl.
Bt-la.
The geology of the l4ires district.
Idem., Vol.
II,
Psirt
1, pp. S<-tS.
136
SCIENTIFIC SURVEY OF PORTO RICO
cao
area,
that some beds
of Eocene
age may
also be
present.
In the
Coamo-Gnayama district which adjoins the Humacao on the west, Hodge
has come
to a
similar
conckision,
basing his
belief on
a specimen
of
Venericardia
alticosta and
large numbers
of foraminifera,
radiolarians,
and diatoms which he fonnd in its uppermost beds.
He thinks that the
lower portion is of Comanchean age.
Mitchell and Hubbard, on the other
hand, working in the western part of the
island, found no evidence of
Eocene
sediments
in
the
older
series.
All
fossils
discovered by them
point toward
an Upper
Cretaceous
age
for
the whole
series.
In
the
Humacao
district,
unfortunately
the
writer
w^as
not
able
to
find
any
fossils that would throw further light upon this problem.
The older
series of southeastern Porto Kico are made up predominat-
ingly of volcanic rocks.
They consist chiefly of
tuffs and andesites, the
former predominating in the western half of the
district and the latter
in the eastern.
The tuffs are largely andesitic in composition, but latite varieties are
also
present.
In
texture,
some
of them
are
sufficiently
coarse
to
be
classed
as volcanic
breccias.
Others contain
sufficient amounts
of
cal-
careous material to be classified as calcareous tuffs and these sometimes
grade into tuffaceous limestone.
The andesites show a rather wide range in texture and mineralogical
composition and include some dacites and latites.
More siliceous felsites
are also present over limited areas.
Augite and hornblende andesites are
of most frequent occurrence and
a
porphyritic
texture
is
usually
de-
veloped in them.
Both extrusive and
intrusive
types
are undoubtedly
represented but it is difficult to differentiate these in the field.
In many
instances, a
parallel alignment
of
the
plagioclase
laths
of
the ground-
mass,
indicating
a
flowage
structure,
is
revealed
in them under
the
microscope.
Sandstones, true
shales, and limestones make up
only
a very minor
portion of the older series in the Humacao district.
On account of the scarcity of outcrops, the rapid alternation from one
type of rock to another, both vertically and horizontally, and the com-
plexity
of
structure,
it was not found
possible
to
subdivide
the
older
series
in the
Huitiacao
district into
a number
of
distinct
formations,
such as has been attempted in some of the other
districts.
Much more
detailed work, than was possible in the
liriiited time at the writer's
dis-
posal, will have to be done to make such a subdivision Justifiable.
The older
series in southeastern Porto Eico has been cut by a great
number
of
intrusive
bodies
of
igneous
rock.
One
of
these,
a
quartz
diorite batholith, underlies the major portion of the Humacao
district.
FETTKE, GEOLOGY OF THE HUMACAO DISTRICT
137
Smaller bodies in the form of
stocks^,
bosses^, intrusive sheets, and dikes,
ranging in texture from granitoid to felsitic and in mineral composition
from gabbro and augite andesite to granite and rhyolite, are also present.
In many instances the areas covered by their outcrops are too small and
too numerous to show on the geologic map.
All of these intrusives are
older than the oldest of the Tertiary beds that overlie the older
series
unconformably along the north and south
coasts
of the island
as they
are nowhere found to intersect them.
The Tertiary limestones and shales, that are
so extensively developed
along the western portion
of the north and south
coasts
of the
island
and which according to Hubbard (1923, p. 66)
are of middle and upper
Oligocene age, are absent from the Humacao
district.
The recent allu-
vial
deposits of the coastal playas and the
flood plains and terraces
of
the
interior
valleys
are
the
only
representatives
of Berkey's
(1915,
p.
10) "Younger Series" within the limits of the Humacao
district.
OLDER SERIES
Tuffs and
volcanic
breccias—The
formations
of
the
older
series
in
the Humacao
district
consist
primarily
of
volcanic
ashes,
tuffs,
and
breccias of andesitic composition, tuffs being the most common.
In the
western half of the
district the major portion
is made up
of materials
of this character, while in the eastern part andesites predominate.
As
a rule, the fragmental igneous rocks are thoroughly indurated so that in
many
instances
they
resist
weathering
even
better
than some
of
the
associated massive types.
Over much of the area igneous rocks, also largely of andesitic compo-
sition, are so intimately associated with the tuffs that separate mapping
is impossible.
On the geologic map, therefore, those areas in which the
former predominate are indicated as being underlain by andesite; while
those portions where the andesites constitute only minor portions of the
formation are shown as
tuff.
It was not found possible to map the vol-
canic
ashes,
tuffs, and breccias separately on account
of their intimate
association with one another.
Gradations from one
type
to the
other,
both vertically and horizontally, are of too frequent occurrence.
In
the
southwestern
portion
of
the Humacao
district
in
the
area
drained by the Llaunel Eiver, beds
of consolidated ash
alternate with
tuff.
Occasionally
volcanic
breccias
are
also
present.
A
thin
section
cut from a greenish gray specimen of the latter type occurring near the
head of the west fork proved to consist largely of fragments of andesite.
These have
a
felsitic
texture and
consist
of an
interlocking mass
of
plagioclase
laths with
occasional phenocrysts
of
augite.
Some have
a
trachytic type of groundmass developed
in them.
Chlorite and epidote
1:I8
SCIKNTlF/t' SL'h'VEY OF PORTft
Uli'i)
are
the
t;lut>f secondary minerals
present;.
The
iiuilrix
i-uii>isr.<
largely
of seeoriclary (|imrtz mid eiilorite.
Microseopie exjmiiiiatioii of n speeimeii
(jl)taiiied near
the head
of
the
east
fork showed
it
to
be
a
bitite
tiiff.
Oraiiis
of
orthnehise,
pkgioelase,
and
aogite
eaii
l)e
ideutilied
in
it.
AJjuiulaut
epidote
has
developed
from
an«-ite and
feld>i)5ir,
and
hirgv
iliuuitities of nceonchiry quartz as well
a,s
a
littU*
serieite are i)reHent
in
the matrix.
The
trdl's of
tiie Patillas Hiver drainage north of
th,(' reser-
voir are also thoroughly indurated and much altered,
(irains of plagio^
idase and
ortli<x-Iase, however, can
still
he
recognized umier
tlu^ mienv
seope.
Epidot«, cldoritc, and
serieite are ahundantly devekjped,
as
well
as large amounts of seeondary quartz.
.\ dark gro(!]iisli gray volcanic
Ijreecia., eoiitaining fragments
ii|) to
11.4
centimeters in diameter from the northern slopes of the Sierra de
C'ayey..
was
also found
to
l)e of
tlie aiuiesitio
type.
Tlie fragments
are
felsitic;
in tc'xture and consist largely of plagioclase latlis in more or less parallel
alignment.
Small amounts
of
needle-like amphihole
are
als<j
prcssent.
The
ande8it(!
is
an.
aiuygdaloidal
varietv,
the amygdules
consisting
of
seeondary
(|uartz
and
occasionally
epidote;.
Besides
the
fragments
of
aiulesite, a few
fel<lsf)ar crystals occur in
the imitrix wddch
is composeel
largely of secondary qnartz anrl epidote.
Thoroughly
indurated
fijie-grained
volcanic
ashes,
alteriuiting
witli
coarser
tuffs, form very prominent outcrops along the upper portion
of
the Turabo
valley,
as shown
in
figure
20.
Some
t)f
tliese
are almost
i<lentical
in. appearance with the Swedhsh
halleflinta.
lender
tlie mi<iro-
FF/tTKE. (iHOUidV OF THE HVMACAO IH^riilVT
j:|{j
scope, they «re sewi tu consist of minote grai]ii4 of quartz, ortlioelase, and
plagiocla^e
togx'thcr with, a
little
pale grcwi
aiirphihole, probahly
«1ov(4-
opeil through inetamorphisoi.
The eoarst^r
tuffs are made up
largely of
anclcsite fragments with
feyitic
textui'e and
tra<:'liytie
Htriicture,
Occa-
sional
plagioclase
pheiiocrysts
af^pear
in
them.
Soiih'
fragments
that
are oompos(!d
of
a
lliic mosaic
of quartz grains
are
also
present.
'Jlu?
fragments
I'ange up
to 2 millimeters
in
diamcd'.er.
The matrix
consists
largely
of
setiondary
(piartz and
epidotc.
Figure
'2'i
is
a
photomicro-
graph, of the ash and figure 32 of the tuff.
Andesite
tuifs are
well exposed
in
cuts and
rpuirries
at
a number of
places
alon,g
tlie nulitary road running southwest
i)et\vccn Caguas
an,d
Cayey.
At K.
-10.9 such
a
tulf crops out akmg Quehradillas creek.
It
coid;aiiis:
fragments
up
to
;^?t-l>
millimeters
in
diameter.
Under
the
microscope they are
seen,
t« consist of andesites, varying in
t(»xtnrc and
,number of phenocrysts as well as in
t,l,ie degree of alteration which
tliey
srjKXTfFie BVh'VEY or porto rico
Iiave uiMlfrgoBp,
Host ut tliem contain plagioelase pheiioerysts wlricli in
iHoiiie
instance:^ have
been
largMily
replac-t'd 'by
.^ecoiidarv
<jnai'tz.
Ocea-
fji(oial
liornbU'iiflc
phciioeryats
are
also
present
in
tliein.
Xuuierons
needles
of ]mk green ampin bole
lja\'e
de^'elope^l
in
ail
of
tliein.
Sec-
ondary quartz has been abniidauily
dej^iji^ited lictween the fragments.
A
short distanee boyinnl
I\. 44 a qiiarr}'
i\)r road
rocrk
lias been opened
iji
a
coar.<e
tuff
euu taiuing fragments
of
aiidesite up
to
5
niillimeterg
in
diameter,
n;'-^''
'^''
v.-hi'd!
''hnw
--onio
rnn!v!iinj\
M S'-r'V'crVHf. exannnation
Fin,
22.
Pfi
If
riimb'f
i-fllle.!/
reveals a felsitic
t.i>xtnre and traehytie strnctnre
in them with occasional
ptienocrvsts
of
plagioelase.
i\fueli
chlorite
ami
a
little
epidotc
ami
sericite liave developed as alteration prodncts.
Fieyoiwl
Iv. 40
\'ery
uiiieli
similar tuffs are exposed.
The
fragmen,ts of an<lesite range in diameter
np
to
7
niillinieters.
In.
textnre
and
structure
they
vary
somewhat.
Some contain nnmerons plagioelase phenocrTsts imbedded
in a ground-
mass of feldspar laths.
Considerable epidotc and
caleite have developed
as seetmdarv minerals
in them.
Otln.'rs
have
a
felsitic
texture and
a
FHTTKE, fJEOLOay OF THE UVMACAO DIBTRIVT
141
tjpical
traeliytie striictirre.
All are amygclaloidal. elilorite usiuiHt filling
the original vesicular openings,
llueli e|ii(lotc and caleite occur lietwecn
the fragments.
Good exposnres of
tutf ami
volcanic
breccias may ako be
j^eeii along
tlie
C'aguas-Juncos
ruad,
jtartieiilarly
in^
tlie
vicinity
of
(Tiirabo, which
lies jnst to the north of the lluniacao
ilistriet.
A
tliiri section cut from
a speeinn^u
olifaijied
southwest
of K.
ti
cou>isis
vl minuTon.s grains
of
plagioelase,
aiigite, and
in'owu^
horuhleiule imbedded
in
a
line
matrix.
ph
<>{
hitili;
fiiff
Situtliirc
miith
f,f T()m:.o
tltc ifetwait
.V<(
]<]pidote has partially replacetl the fehlspar grains.
Between
Iv. 13 ami
K. W, a small cpiarry has heeu opened
in a
tulfaeixnis limestone
inter-
bedded
witli
tJici
aude.sitic
tulfs.
Altout
30
j'eet
of
the
i'onner
arc
exposed.
Soutliwest
of the magnetite
deposit,
stmth
of Torres,
a'ttdii bed
of
latite
tuf! occurs
associated
with
daciies aud
aiidesites,
a
])hotoonero-
grapli
of which
is shown in
figure
23,
It
is composed
of
raggnni and
142
SCIENTIFIC SURVEY OF PORTO RICO
rounded grains of
plagioclase^
orthoclase, and a
little
augite imbedded
in a fine matrix in which some pale green amphibole and chlorite
caji be
distinguished.
A little magnetite
is also present.
In the
eastern portion
of
the
district the
greatest
area
of predomi-
natingly tuffaceous rocks lies to the north of the Humacao playa.
Some
of these
consist
of consolidated ash beds.
Numerous grains
of plagio-
clase
are
usually
present,
some
representing
broken
fragments
while
others
are complete
crystals.
Small amounts
of
orthoclase and
quartz
also occur in them.
In many instances the feldspar has been more
or
less
completely
altered
to
epidote,
chlorite,
calcite
and
sericite.
The
matrix,
as a
rule,
is extremely
fine and
consists
of
sericite,
kaolinite,
and
calcite—the
latter often being very abundant.
Coarser fragmental
volcanic rocks are interbedded with the ashes.
The fragments in almost
every instance consist of andesitic material.
Thoroughly indurated volcanic ashes of the hdlleflinta type, resembling
those along the upper portion of the Turabo valley, also occur along the
Santiago valley, north of Naguabo.
Plagioclase grains and minute frag-
ments
of well-rounded
andesite
can
be
recognized
in them under
the
microscope.
Epidote and needle-like
crystals
of amphibole can be
dis-
tinguished in the fine matrix.
Where
the tuffs come
in contact w4th
the major
intrusive
of south-
eastern Porto
Rico,
the San Lorenzo quartz
diorite batholith,
they
are
usually metamorphosed
into hornblende
schist.
Similar
changes
have
also taken place in them in the vicinity of the magnetite deposits of the
Juncos-Torres region.
These are described under contact metamorphism.
Andesites.—Andesites rank next to the tuffs in order of abundance in
the older series.
They show a rather wide range in texture and miner-
alogical composition and include some
dacites and
latites.
Augite and
hornblende varieties are of most frequent occurrence, but some that are
largely fekl spathic in composition are
also present.
A porphyritic tex-
ture is commonly developed in them.
Both extrusive and intrusive types
are undoubtedly represented but
it
is
difficult
to
differentiate
these
in
the field.
In many instances a parallel alignment of the plagioclase laths
of the groundmass, indicating a flowage
structure,
is revealed in them
under the microscope.
Andesites are almost everywhere associated with
the
tuffs, but on the geologic map an attempt has been made
to show
only the larger areas.
Andesites
crop
out
along
the
headwaters
of
the
west
fork
of
the
Patillas Eiver along the southern slopes of the Sierra de Cayey.
These
range in texture from
felsitic to porphyritic, the former type predomi-
nating.
When phenocrysts are present they consist of plagioclase.
Feld-
FKTTKH, ai':(flJH!Y OF THE JIL'.UACAO DWTHICT
J 43
spar laths
iii'<- the
chi<'f constituents
of the inatrix and m«Miietite
is the
moi^t important accessory mineral.
Clilorite,
sericiie, and imditc arc the
pri,n('i|)al secondary minerals along witli occasional
nests
or
hunclies
of
cpidote.
Here and
there
tJie
anxlesites are cut by vcinlets
ol" secondary
(juartz.
Some
pyritc
has
been
jji trod, need.
An
andesite
(sf
the
honi-
hlcndic va.riety occurs on the ojtposite slopes of the Sierra de C'aycy, with
an
area
of
tuff
intcrvcnin*;-.
Plagioclase and
a
light green horTd>lende
oecnr
as
phcnocrysls
in
it.
The matrix
consists
chiefly
<d*
plagioclase
laihs with minor amoujits of magnetite and exhibits
a
typitnl trachytie
structure.
East of
tlie
bridge
at
K. 4L ahrng
th,e
road
southwest
of Caguas,
a
hornblende andesite porphyry crops along the bed ul a small stream.
A
photoniicrograpli
of
tliis
rock
appears
in
figure
24.
TJie
[ilienocTysts
consist cduefly
of
plagioclase, now almost
ctniiplctcly
altered
to
scrieite
and
chlorite.
Occasional
er^^-^tals
of
bluisli
green,
horidilende,
in
part
altered
to
chlorite,
are
also
prescjit.
The ground mass
is made up
of
144
SCIENTIFIC SURVEY OF PORTO RICO
plagioclase
laths and
hornblende
in more
or
less
parallel
alignment.
Magnetite and a little quartz are the chief accessory constituents.
About
850 feet southwest of K. 41 a trachyte occurs on the south side of a small
stock of quartz monzonite that intervenes.
It contains occasional pheno-
crysts of orthoclase.
Plagioclase and quartz are only sparingly present.
The groundmass consists mostly of orthoclase feldspar, together with a
little plagioclase and occasional flakes of biotite.
Numerous spherulites
of orthoclase occur in
it.
Between K. 41 and K. 42, a short distance up
Quebradillas Creek, a quarry for road rock has been opened in a horn-
blende
latite.
Phenocrysts of orthoclase and plagioclase are prominent,
the latter in part altered to epidote.
Crystals of pale green hornblende,
almost completely altered
to
chlorite,
are
also
present.
The matrix
is
made up of innumerable small feldspar laths.
Epidote,
chlorite, and a
little
calcite occur
as secondary minerals.
A short distance further up
the creek a fine-grained rock with only occasional phenocrysts
of ortho-
clase and plagioclase and with some quartz
besides
feldspar and
occa-
sional
biotite in the matrix crops
out.
On up Quebradillas Creek
out-
crops
of
andesites,
dacites, and
latites may be
seen
interstratified with
tuffs and volcanic breccias.
Andesites and andesite porphyries occur along the road between Caguas
and San Lorenzo.
A thin section of a specimen obtained a short distance
west
of K.
5
contains numerous
plagioclase
phenocrysts
which
show
maximum extinction angles of 21
degrees
at right angles
to the
albite
lamellae, and are, therefore, i^robably andesine.
The groundmass consists
principally
of
feldspar
laths
arranged
in
trachytic
fashion.
A
little
chlorite and occasional nests of epidote occur in
it.
iVndesites predominate in the older series in the northeastern portion
of the Humacao district.
Many different types are present representing
feldspathic,
hornblende, and
augite
varieties.
Some
have
felsitic
tex-
tures while others are porphyritic.
In many instances they show under
the
microscope
a
parallel
alignment
of
the
plagioclase
laths
of
the
groundmass, indicating a flowage structure.
Ad
augite
andesite
comprises
the
footwall
of
the magnetite
deposit
east of Juncos at its west end.
It possesses a felsitic texture with a ten-
dency for the many feldspar laths of the groundmass
to be
in parallel
alignment.
A few
phenocrysts
of
plagioclase
of an
andesine
or
acid
labradorite variety and a little augite are present.
The groundmass con-
sists chiefly of plagioclase.
Titanite
is an accessory mineral.
Pale green
amphibole
aggregates
occur along small
fractures
as
well
as
in
places
once occupied by
augite phenocrysts.
The
plagioclase phenocrysts
are
clouded by alteration products.
Beyond the andesite tuff, adjoining the
FETTKE, GEOLOGY OF THE HUMACAO DISTRICT
145
magnetite deposit on the northwest, there
is an occurrence of hornblende
andesite porphyry.
Augite phenocrysts with reaction rims of hornblende
are prominent.
The groundmass
is made up predominantly of a brown-
ish green hornblende with
only minor amounts
of
plagioclase.
It
is
coarse in texture.
A little magnetite and apatite are present.
North of the calcareous andesite tuff that overlies the second magnetite
deposit southeast of Juncos an augite andesite with felsitic texture crops
out.
Under the microscope a few large phenocrysts of augite can
still
be recognized.
Others have been altered into aggregates
of epidote
or
brownish green biotite and secondary quartz.
The groundmass
is com-
posed of plagioclase laths in more or less parallel alignment, biotite, and
augite.
The limestone, with which the third magnetite deposit
is
asso-
ciated,
is
also overlain by an
andesite.
Southwest
of
this
deposit,
be-
tween it and the quartz diorite batholith, a hornblende andesite porphyry
occurs.
Plagioclase and pale green hornblende are present in it as promi-
nent phenocrysts.
The groundmass
consists
of plagioclase, hornblende,
and magnetite.
The
lath-like crystals of the former show a
slight ten-
dency toward parallel alignment.
A feldspathic andesite comprises the hanging wall of the fourth mag-
netite deposit southeast of Juncos.
In thin section,
it
is seen to possess
a
felsitic
texture and
distinct
flowage
structure
brought
out
by
the
parallel alignment of the plagioclase laths.
A few phenocrysts of plagio-
clase and
a
little
light green hornblende
derived
possibly from
augite
are present.
Occasional aggregates
of epidote
also occur.
The matrix
consists predominantly
of
plagioclase
laths.
Skeleton
crystals
of
iron
oxide,
possibly magnetite,
are prominent.
A
little hornblende
is
also
present.
South of the magnetite deposit at the head of the Anton Euiz
valley prominent outcrops of an augite andesite porphyry occur over
a
considerable
area.
The phenocrysts
in
it
consist
of
pale green
augite
and plagioclase.
The matrix
is composed of plagioclase
laths, showing
some tendency toward parallel alignment and magnetite.
A
little pale
green hornblende and epidote have developed in
it as alteration products
from
the
augite.
Andesites
and
dacites
occupy
tlie
1)elt
between
the
above magnetite deposit and the one south of Torres.
A hornblende andesite crops out on the south side of the Gurabo valley
in the vicinity of Torres.
This contains a few phenocrysts of greenish
brown hornblende imbedded in a groundmass of plagioclase, hornblende,
and magnetite.
A faint flowage structure
is noticeable.
Small amounts
of
epidote and
calcite have formed
as
alteration
products from horn-
blende and a little sericite from the feldspar.
An augite andesite and a latite occur between
a bed
of limestone and
146
^"^CIENTrFIC PURVEY OF PORTO RICO
the outcrop of magnetite south of Torres.
Whether these are extrusive
or
iiitrnsive
sheets could not
be determined from
their
field
relations.
Tlie andesite wliich immediately overlies the limestone, in thin
section,
shows a
parallel alignment of
its plagioclase
laths indicating
a flowage
structure.
Tlie latite contains phenocrvsts of plagioclase and orthoclase
in a groundmass consisting chiefly of plagioclase laths and
a
little horn-
blende.
Small amounts
of epidote occur
as
alteration
products
in
the
groundmass and also along fractures.
(Alts along the road up Pena Pobre valley expose feldspathic andesites
at several places.
One of these has plagioclase
latlis, small amounts of
hornblende, and a
little magnetite imbedded
in
a partly glassy matrix.
The rock^ in thin
section, shows a pronounced flowage structure, due to
the
parallel aligiiment
of the feldspar
crystals.
The rock contains nu-
merous small fragments of a pre-existing andesite imbedded in
it.
The
latter
contain
feldspaper
phenocrysts
in
a
somewhat
finer
textured
groundmass
than
the main
rock.
Very
similar
andesite may
be
seen
along the Blanco
valle}^ above
its junction with the Pena Pobre.
Here
also
in one
instance, small fragments
of pre-existing andesite were
ob-
served
in
a
feldspathic
augite
andesite.
Hornblende
and
feldspathic
andesites are the pi'edominating rocks along the Santiago valley.
At El Morillo Point
a feldspathic andesite forms the low
hill
rising
above
the
play a
level.
It
contains
occasional
phenocrysts
of
andesine
feldspar and hornblende in a groundmass of plagioclase laths and horn-
blende.
The former
are
in more
or
less
parallel
alignment.
Similar
andesites occur along the Humacao-Xaguabo road
to the north.
Along the shore southeast of Naguabo Playa, andesite porphyries form
the rather prominent
clifl's that have been cut back by the
waves.
An
augite variety contains prominent phenocrysts of augite and labradorite
in
a ground nuiss
of
feldspar.
Uralite has developed along the borders
and
cleavage
cracks
of
the
augite and
epidote
occurs
along numerous
minute fractures that intersect the rock in
all directions.
It shows only
a faint flowage structure.
A feldspathic hornblende variety contains nu-
merous phenocrysts of plagioclase and a few of bluish green hornblende
in
a groundmass made up
chiefly
of minute
feldspar
laths.
Both
the
plagioclase phenocrysts and
laths
are
in parallel alignment.
Some
epi-
dote has developed from the feldspar and epidote and secondary amphi-
bole occur along minute fractures in the rock.
Andesites and andesite porphyries crop out at a number of places along
the road leadiiig northeast from N"aguabo Playa.
At K.
(V2.2
a quarry
for road rock has been opened in andesite.
It lias a felsitic texture and
a
flowage
structure.
The
groundmass
contains
not
only
occasional
FETTKE, GEOLOGY OF THE HUMACAG DISTRICT
14;
phenocrysts
of
plagioelase
but
also
small
fragments
of
pre-existing
andesite.
Felsite.—At
several widely
separated
localities
in the Humacao
dis-
trict small areas of dense usually light-colored felsitic rocks, more siliceous
than the andesites described, occur in the older
series.
Their exact rela-
tionship
to
the
other members
of the
series
could not be
determined.
They may
re^Dresent
intrusive
bodies
or
possibly
they
are
lava
flows
interstratified with the tuffs and andesites.
The more prominent of these
occurrences
are
show^n under the heading
of
felsites on
the
geological
map.
They crop out on both sides of the Patillas valley south of Patillas
and along the coast west of Naguabo Playa^ as well as at several places
along the road leading northeastward from the latter point.
Along the west bank of the Patillas River where the road from Patillas
to Arroyo
crosses
it,
a quarry
for road material has been opened
in
a
rock
of
this
character.
It
possesses an extremely
fine
felsitic
texture
and light
color.
Fractures intersect
it in
all
directions and
it
is cut by
several
dikes
of
rhyolite
porphyry
showing
prominent
phenocrysts
of
quartz.
Under tlie microscope^ occasional small phenocrysts of orthoclase
are noticeable
in
a
fine groundmass composed
chiefly
of orthoclase and
quartz.
A little plagioelase is present as a minor accessory.
I'he feldspar
is cloudy from alteration.
This rock apparently
is a rhyolite.
South of
this locality along the west side of the Patillas valley a number of out-
crops of very similar rock may be seen.
Microscopic examination shows
them to range in composition from rhyolite, through trachyte and
latite
to dacite.
On the east side of the road leading southeastward from Patillas, about
half w^ay to the coast, the contact between a dacite and rhyolite, in many
respects similar to those noted above, can be seen in a small quarry.
The
dacite which overlies the rhyolite has a
felsitic texture with a few small
phenocrysts
of plagioelase imbedded
in
a groundmass composed
essen-
tially of plagioelase and
quartz.
Minor amounts of magnetite occur as
an accessory constituent.
A little sericite and chlorite are present in the
groundmass
as secondary minerals.
The
rhyolite
is
considerably more
brecciated.
It also has a fine felsitic texture but contains no phenocrysts.
Under the microscope,
it
is seen to consist of an allotriomorphic
inter-
growth of quartz and orthoclose.
The orthoclase in part has been con-
verted into sericite by alteration.
AVest
of Xaguabo Playa
along the coast
a
fine-grained
dense
dacite
forms the cliffs.
Under the microscope it is seen to consist of plagioelase
and quartz.
Chlorite,
calcite, and epidote have been developed in
it by
alteration.
Along the road to the northeast of i^aguabo Playa between
]4,,S
HI'IENTIFIV tiVIlVEY OF
/'OKI'O
lUf'O
K. OT
ajitl
K.
1)8, a
liglit (jolorcd, dense, badly fraetured
rrn-k, very simi-
lar
to
that
w«'f,t
til'
rrttillas,
crops
out.
A
few
seattHred
pheiiocryBts
oit'irr
in
a grouiidmass composed
csseulially
of
orHioela^c and
quartz.
Titairite, fjlagioclase, and lioriiblende are accessory constituents.
A pliol:o-
mierograpli of tlnH rock is sliowii in figure 25.
Between K. (iO and K. (51
a very similar rock has been exposed in a small quarry for road
in;ilcrial
It contaijis a
few small pbeiiocrysts
of orthoelase
in a
<4'i"«>"iidjimss con-
nisting
of
an
allotrioniorphie
intergrowth
of
orthoelase
and
quartz.
Orth,
Flagioelast!.
biotite,
nuignetite,
and
apatite
are
pre>eiit m
luiuor
ac-
cessories.
Linie.HtoHe.i.-—True liiiiestones
art; of comparatively
raie
ui-i-uj-reiice
in
the older scries of the Ihiinacao
district.
Those that
were encountered
during tbe course of the ])resi'nt
investigation, had
recryslallizcd
to snob
an^ extent
that
all
traces
of organic remains were
oblitc-ratcd.
Even
in,
these
limestones,
in
sojne
instances,
small amounts
of
tull'aecoUH
uui-
FETTKE, (JFjOLOGY OF THE HUMACAO DISTRICT
149
terial can be detected.
In manv cases the
tuffaceoiis material predomi-
nates
so
that the
rock becomes
a
calcareous
tiifr, which may
in
turn
grade into an ordinary andesitic
tuff.
In
still other instances, andesitic
tuffs have been so completely altered to carbonate that they may readily
be mistaken for true limestone.
Collores
limestone.—The most conspicuous outcrops
of limestone ob-
served in the Humacao
district occur along two belts
; one of which be-
gins
11/^ miles
east of Juncos and extends
S. 53°
E.
for
a distance of
about
6 miles.
It has been called the Collores limestone on account of
its typical development in the barrio of Collores de Piedras.
The dip
is
toward the northeast and ranges from 32 to 41 degrees.
The second Ijelt
was traced for only a short distance.
It lies about 7/10 of a mile nortli-
east
of the
southeast end
of
the
first and apparently
dips toward
the
soutliwest, although the character
of
its outcrop
is
sucli
as
to make
it
impossible
to determine
this
positively.
This makes
it appear
that
a
syncline
is present and
that
the two
belts may
be formed by outcrops
of the same bed.
xVlong the southwest
belt, the
best development of the
limestone ap-
pears in the barrio of Collores de Piedras.
The limestone has
a
strike
of X. 50° W. and dips 32° toward the northea(St.
Prominent ledges of
gray and white
crystalline
limestone,
in some
instances 40
feet
thick,
crop out.
The total thickness is estimated to be 250 feet.
A tlrin section
of the crystalline gray limestone, a pliotomicrograpli
of whicli
is sliown
in figure 2G, reveals an interlocking nu)saic of calcite crystals of medium
texture with only occasional minute grains of
iron oxide anu)ngst tluMu
and a little chlorite.
Qualitative analyses indicate a verv ])ure limestone.
There
is very
little residue after treatment with cold dilute hydrocliloric
acid and only
slight amounts of iron and alumina and a trace of mag-
nesium carbonate are present.
In the vicinity of the magiu^tite deposit,
whicli occurs in
its upper portion, the limestone has been converted into
an aggregate of calcite, garnet, pyroxene, chlorite, quartz, magnetite, and
specular hematite.
These clianges in the limestone are referred to again
under contact metamorphism.
At
the northwest end of the
belt,
just southeast of the Juncos mag-
netite deposit, the limestone has been completely metamorphosed into an
aggregate
of
garnet,
pyroxene,
amphibole,
epidote,
clilorite,
and
talc.
Only minor amonnts of calcite are
still present in
it.
x\t the southeast
end, 1500 feet northwest of the most southeasterly of the magnetite de-
posits shown on the geological map, prominent ledges of white crystalline
limestone can also be seen.
Some garnet rock is associated with them.
Along the northeast
belt, outcrops of limestone were ol)served at only
150
.SCIEXTIF/C ^I'RVEV OF PORTO
h'ICO
one
laertlity
iiboiit 9/Hl of a
mile south
oC
Torres,
it
is
a dark gray,
fine grained
variety which
litis not
been
nietrtinorphosecl
to
the
extent
that
the
linie^^tojie
of
the
first belt
has.
Jn
thin^
seetion,
it
is
seen
to
consist prineipaJly of ealeite, hot occasional grains of
(itiartz, angiie. and
l^lagioclfisc are also present,
Tbe lalter nndonbtedJy represent tntTaceoirs
Mnitcrial
lieposited
.sijnnJtaiieoiisly
with
the
ejileareons
coostitiients,
A
few small grains of pyrite are
also present.
Figure
"il' shows a
photf)-
niicrogra|)li
of
tliis
limestone.
Southeast
of
tliis
limestone ahmg
the
strike,
a
pixnninent
ledge
of metamorphosed
roek
was
noticed
at
one
place,
wtiieh, under
tlie jnicroseope, was foiunl to consist ahnost entirely
of an
aggregate
of garnet grains.
A
Jittic
pale green amphihole and
deep brown
Iriotite occur betweciu
tlie former.
This rock apparently
rep-
resents metamorphosed portions of the limestone.
TntTacerms
limeston,e.s
am]
calcareous
tiilts.— Sontlieast
of
I\.
12,
along the Jnneos-Caguas
roail northwest of the town of dnncos, a small
h'HTTKK, ilFJUJtCY OF THE Hl'MAFAtt DlHTUIVT
1.31
qiiiiriT
ha,s been opened
in
a
tufrawous limestone
iiiterl)e<lcle<l
with
eal-
(•rti'eons tullB.
The furnuition.s rtrike X.
42'''
E. ami dip
73"' toward the
sontlicttst.
About
:iO
feet of limestone mv. expose*!
in the
(|iiaiTy.
The
upper beds consist ot a light g-reo rush -gray fine-grained
frHgnneutal roek
wtiieh, under
the mieroseope,
is
seen
to
be made np
chiefly
of eaknte,
serieite.
chlorite, and kaolinite.
i)eea<iorial remnants of
[dagioehise can
'ho detected in
it.
This roek may be classified as a tuffaeeoirs limestone or
perliaps
Ijetter.
a
ealeareous
ash.
Below
it
oeeurs
a
sojnewhat
coarser
Qtz.
greenJsh-gray
hreceiated
tufl*.
Caleite
has
rephieed
ahnost
the
entire
matrix
of the
tuff,
but
original
structures can
still
l»e
detected
in
it.
Chlorite now
oeciipies
the
space
wdiere
fel(ls])ar
graijis were
originally
present.
Only oceasiomil
remnaid:s
of
plagioelase
crystals can
be
seen.
Sinrihir calcareous
tnlfs
were
observt^d
in the
tn,ft' areas
souAli
of the
Anton
Rniy. vallcsv near
its low'er end and north of Arroyo and
Patillas.
Shales.—Sljales
are
of practically
negligible
isuportanee
in
the
older
series
of
the Humacao
district.
All
of
thosti examined bv the writer
152
SCIENTIFIC SURVEY OF PORTO RICO
contained so much material that
is clearly of volcanic origin and were
so
intimately
associated with
tuft's
that no attempt was made
to map
them separately.
They are included with the latter on the geological map.
Sandstones.—Sandstones
are
of only minor
importance.
An
occur-
rence was observed at the east end of the Patillas dam.
The rock has a
mottled dark brown color and^ under the microscope,
is seen to be made
up chiefly of quartz grains reaching a maximum diameter of
.32
milli-
meter and an occasional fragment of some pre-existing rock.
The bond
between the grains consists of chlorite, hematite, and sericite.
The out-
crop
is not extensive and probably represents a small lens interstratified
with the volcanic tuffs and breccias which crop out at other places near
by.
At the dam, the contact between
it and
a monzonite porphyry
is
exposed.
Along the upper portion of the west fork of the Patillas River, promi-
nent ledges of medium-grained quartzite crop out along the creek bed.
Microscopic examination
" reveals quartz
to
be the
chief constituent but
orthoclase and plagioclase are also present.
A little chlorite, calcite, and
sericite occur between the grains of the former minerals.
INTRUSIVE8 IN THE OLDER
SERIES
Xumerous intrusives, ranging in composition from augite andesite and
gabbro on the one hand and to rhyolite and granite on the other, occur
in association with
the older
series of the Humacao
district.
Some of
these are present as intrusive sheets or
sills varying from a few inches
to several hundred
feet
in
thickness,
while
others
are
in
the form
of
stocks and
bosses, ranging from
a small
fraction
of
a mile
to
several
miles in largest horizontal dimension.
One of the latter type has a
suffi-
ciently large area! extent to be called a batholith.
Innumerable dikes of
widely different mineralogical composition cut across, not only the forma-
tions of the older series
itself, but many of the igneous rocks intrusive
in
it as well.
As has already been pointed
out,
all of these manifesta-
tions of igneous activity antedate the middle Oligocene.
INTRUSIVE SHEETS AND SILLS
The numerous inirusive sheets and
sills^ that are
interstratified with
the
tuffs,
lavas and sedimentary rocks of the older
series,
are,
for the
most
part, composed
of
augite,
hornblende,
and
feldspathic
andesites
and
andesite
porphyries.
Somewhat more
siliceous
varieties,
such
as
dacite and latite, are of only minor importance.
They range from a few
inches
to
several hundred
feet and perhaps more
in
thickness.
In
the
field, on account of the limited occurrence and extent of outcrops,
it
is
FETTKE, GEOLOGY OF THE HUMACAO DISTRICT
153
very
difficult and
often
impossible
to
distinguish them from
extrusive
types of similar mineralogical composition with which they may be
as-
sociated and
they
have,
therefore,
not been mapped
separately.
It
is
believed that
these igneous masses were intruded
into
the
older
series
contemporaneously with
their deposition or at least very
shortly there-
after while the beds were
still in a nearly horizontal position and before
the orogenic movement set in that produced the present folded structure.
STOCKS, BOSSES, AND BATIIOLITHS
The stocks, bosses, and batholiths of granitoid igneous rocks that have
invaded
the
older
series
of
the Humacao
district
are
clearly
of
later
origin than ihe intrusive sheets, and
sills just described, as they intersect
the latter as well as the
tuffs, lavas, and sediments with which they are
associated.
They, therefore, belong to a somewhat later period of igneous
activity, probably contemporaneous with the period of folding and uplift
or
else
closely following
it.
The more important types represented
in
this group are gabbro, quartz diorite, granite, and quartz monzonite—the
gabbro being the oldest and the quartz monzonite the youngest.
Juncos (jabhro.—A small intrusive mass
of gabbro apparently
in the
form of a stock, occurring 1^ miles northeast of Juncos,
is the oldest of
the second group of intrusives.
Its outcrop covers an area of probably
not more than 4000 feet in maximum and 2000 feet in minimum dimen-
sions.
The rock has a medium granitoid texture and .under the micro-
scope
is found to consist chiefly of plagioclase of the labradorite variety
and
augite.
A
little greenish-brown hornblende
occasionally
occurs
as
reaction rims around the
augite.
Titanite
is
the
chief
accessory
con-
stituent.
The Juncos locality is the only place where a rock of this type
was observed.
San Lorenzo
quartz
diorite,—The major
intrusive
of
the Humacao
district consists of a medium to moderately coarse granitoid rock possess-
ing the mineralogical composition of a quartz
diorite.
The largest out-
crop underlies a rudely rectangular block approximately
15
miles long
and 12 miles wide, which extends from Juncos on the north to Maunabo
on the south and from the mouth of the Humacao Eiver on the east
to
the headquarters of the Del Espino on the
west.
Exposed over
a con-
tinuous area of over 180 square miles,
it
is evidently of sufficient size
to
be called a batholith
(Daly, 1914)
rather than
a stock and
will,
there-
fore, be designated hereafter as the San Lorenzo batholith, since typical
exposures occur in the vicinity of that town, particularly along the high-
way
leading eastward
toward Las
Piedras and
along
the
Del
Espino
valley
to the south.
The bed of the latter stream
is strewn with great
151
Hi'lEXTU-'IC HFRVEY OF I'OUro Rid
lRnil<lf»rs of
tlie
{|iiHrt/. diorite on some of which apjH'jir criide
desigiij^. as
shown
ill
ligiirt' 28.
<rarve<l upon tliem by the
aborigiiuil
ijilinbitmits
ol'
l*ort.o Bico.
Huge roiunhHl iiiouoliths, such
as are hIiuwu
in
figure
21>,
form eonspieuous outcrops in the quartz diorite area.
The highway lead-
ing across
i'nnduTas liidge from Mauriabo
to Yahucoii nho aifonk exeei-
e)it «)pportiinities to examine the quartz diorite at
eloi^e range.
A similar
(juarty.
diorite crops out abng
the
(luralio
valley
uortht'ast
of
Junc'os and
als<i along
the upper portions
of the
lihmeo and
l^'ena
Puhre
valleys
in
the nortbern
portion
of the
Ilunuu.'ao
district,
''J''lH^-*e
areas are a part of a large stock which
underlies
a considerable
pindJcrn
of
\hr
f.sNiuillo Rnnp-
;ii!-l
uIihIi
e^
ni,.!uuhh.,iiy
r::hn>.-:.^d
is!
dm
uini;
'^^:AJ,
(l<']}th with the main
Saii Lorenzo batholith.
Another similar but some-
what smaller stock occurs in the vicinity of (Jaguas.
^!ucli smaller satel-
litie stocks of from
'^4
to % of a mile
in diameter were ol)served along
tlie cast fork of the Lbrunel Kiver, along the military road about
'4
jniies
soulliwest of Caguas, and about % mile east of
tlu' main
hathtilitb, east
of Las I'iedras.
I'luter
tlie
ml(!rosco|ie, the San Lorenzo quartz
diorite
is
seen
to
he
composed
principally
of plagioclase
of an
andesiue
or
acid
lal>raflorite
variet}', quartz, and a dark brownish -green liomble.nde.
Orthoclase,
hio-
tite.
occasionally
augite,
magnetite,
tilanite, and
apatite
are
the
chief
accessory constituents.
The rock, as a rule, possesses a motleratcly coarse
granitoid
texture.
This
phase
is by
far
the
predtunimiting
type,
but
sometimes
(piart'K drops out almost entirely so that
the
rock he<M)mes
a
Fh'TTKh\ (IFJiiJXiY
(H-' THH HtW/AALiO nJSTh'If'T
15
diorite;
iji a few otlier
ijistrtiirt'S orthoelase
takes^
tlie place of plagioekse
procJocdrig a limiibleiMlo g-rnnite: and
in rare eanes
aiigite replaces
liorii-
bleiide m
that the
roek k
a
>r«l»l)n).
Oeeasioiml'ly,
in
the
case
of
tlie
sinaUer
satcllitiji
stock!>, the texture m porphyritic.
Figure
-10 shows a ]jhotoiiiicrograpli of th.e (piartz dioritc from a spcei-
men
obtained from an
mitf/rop
.south
of
the Ban
LorenKo-Las
Fiedras
roiu]
al)out B^X. miles
!>iout;lieast of
tlie former town.
Playioehi.se, quai'tx,
and liornhleiid(!
art; the iIhhh' chief constitnent.s present.
The phigioehisu
shows maAinium
extijietioji
angles
of
25
degrees
in
seetiou!^
at
right
angles
to
the
allute
lamelhn and
frequeiitlv
exhibits
pronoiun^ed
zonal
growtli.
The liornblenth;
is marl<erlly pleuchroic, ranging from yellowish
browji
throngli
greenish
brown
to
deep
brownish
green.
Ortluxdase.
l)n:)lite,
jnagiielite,
titanite,
anil
apatite
are
th.e
chief
accessory
const it^
iients.
The order of
erystalli'/.ation
of
thi;
various ndnerais appears
to
have been apatite, nnigiietite. titanite, iun-nblejnk^ plagiorlase, ortJiocbisc,
and
(puirtz.
A
little
sericite
has
develoi)ed
from
the
phigio<'lase
aiul
small amounts of chlorite and
ei)idote fr(uii
tln> biotitc.
An augite rliorite phase of the 8an Lm-enzo ipiartz diorite occurs ah>ng
the nortliern
slopes of J^^mduras Ridgt!
abcuit
2
ndles
t^ist of 'Yabucoa.
Figure 31 shows
a
jdiotoinicrograjdi
of this rock.
Hornhhmde,
plagio-
clase, and augite are the
principal
constituents.
The hornblende
is
in-
tensely pleoehroic, ranging from yellowish brown through greenisli brown
to brownish
gri'cn.
Tbe
plagiochnse
sliows maximum
extinction
angh's
of 35
degrees
in
sections
at.
right
angles
to
tlie
albite
bunelbn.
The
augite
freepiently
is enveloped
l>y
reaction
rims
of
hornblemje.
Basal
150
,srlEXTIFI€ Sl'RVEY OF PORTO KIPO
f^wtiouH of fiiiirite .sniTimiHletl by
liorJihJt?ii(lt' may be seen
iii
tbiii
heetioii,
eadi
witli
lifi
typical
prisniiitic
cleavage
(levelope<l.
liitcrgrowtbs
of
aogite and
bjsrnblende
arc
also
present.
Plagioelii.se
appears
to
have
crystallized before hornbleude, inclusions of the former occurring in the
latter.
The
latter
al.so
i'recpieutly
tills
the
space
between,
plagh)cbii^(!
crystals,
IMagnctiic,
orthoelase,
titanite,
anil
apatite
are
the
principal
aceessorv const itiients.
South
of Ilnmacao,
11/^
miles,
a
diorite
phase
develcjped.
At
tltis
f»oint
tiie
chief
constituents
arc
plagioclasc,
:fre<piently
sliowiiig
zonal
gro'^v'th
with niaximum
extinction,
angles
o,f
2()
degrei>s
in
sections
at
right angles to the albite
lamella?, and
an, intensely ])leoebroic brownish
green
liornithi-mle.
Quartz
is only
a
Jiiinor
ejuistitueuf..
Ttic
other
ac-
cessory minerals are orthodase,
biotite, magnetite,
apatite, and
titanite.
A little sericite has developed from
tlie fchlspar and chlorite and epidote
from the
hi<4.ite.
One
,mile south
of Humacao
a gahbro phase
occurs.
Plagioelase ami aogite are
its main
c,-o,rnpoi]ents.
The plagioclasc shows
FHTTKE. (lEOJjmY OF THE
II VMACAO IHBTRIOT
157
jiiHX 1111 mil
I'xriiietioji
angles
itf
-iSV,
iU»«;ret'H in
seeliuD.s
sit
right
angle?
t(> the albite
himcliii'.
The augiti;
is a pale gTeenish-gniv iHiii-phwchroic
variotj.
Jt sometimes contains iiiternal growths of dark brownish green
hoi-jibleinlc! in paraUel orientation
with, one another, and
is
neeasionall}-
sHrre»iinde<l
\}x narrow rims of the .^anic mineral.
Magnetite and ortho-
elase
a,re
tlie otlier accessory minerals besides hornljlenxh^
The
orcjcr of
<;rv8tHliizHtion appears
to have been magnetite,
angite, and
plagioeJase,
Firj.
31. ^--^^Pl*.
Irs caM
<jf Yalimm
aJiliough
<ieeasio]ial ijudiisions
(»f small plagioelasc crystals in
tlie augitc
indicate that some of the former antedates the
aiigite.
Alojig the headwaters
of
tlie Ingenio
Iliver.
in
the
Yicintty
of Teja
Piedras
barrio,
the
(piartz
diorite
passes
into
a
hornblende
granite.
W ieroclijn:*, orthoidase,
cpiartz,
lioriihh»:nde. and
hiotitc are
its
eliief eon-
stitnei)t,s.
(:rrap]iic
intergrowtiis
of
rpiartz and
ortlniclase
occur
in
it.
Plagioelasc
is of only minor
im}KirtaiH'e.
The other ancessory minerals
are magnetite,
titanite, and
afjatite.
A
little
chlorite and
cpidote have
develo|)ed from, the biotite.
158
t^CLENT/FlC .sr'KVNY OF
I'OliTd
RK.'O
AH
of
the
above
varieties
are
merely
tliffereit
phaneH
of
tlie
Sail
Lorenzo
fmirrfi
rliorit-*.
Nnwlirro
is
t1u'r«»
eviflftu'c
that-
they were
in-
truded as distinct
iiiiiis.
(hi
ihr
<A\n'v
hnsid.
i nsn-il i-n^sl
.lusiiges from
one tjpe to the other may frequently
be
seen.
They
represtnit segrega-
tion or clilTerentiation phenomena that took place
iji the main mass alter
.it had been intruded in its present position.
Tilt Luqnillo
stock
con,sists
of
a
quartz
diorite
almost
identical
in
Mineral composition and texture with
tl:ie main
.San Jjorenzo mass.
It
FETTKE, OEOLOdY OF THE HUMACAO DISTRICT
159
is particularly well exposed
at the
Falls of the Blanco River, shown in
figure
32.
Plagioclase^ quartz, hornblende, and
biotite are
its principal
constituents,
while
orthoclase, magnetite,
apatite and
titanite
occur
as
accessory minerals.
The
plagioclase shows maximum extinction angles
of 301/2 degrees in sections at right angles to the
albite lamellae.
Zonal
banding
is common
in
it.
The hornblende
exhibits pronounced
pleo-
chroism,
ranging
from
yellowish-brown
through
greenish-brown
to
brownish-green.
Biotite
is not always an
essential
constituent.
Along
the north side of the Pena Pobre valley
it
is practically absent in places.
The Caguas
stock
is composed
of
a
grano-diorite phase
of
the San
Lorenzo quartz
diorite.
Plagioclase,
quartz, hornblende, and orthoclase
are
its
essential
constituents.
The
plagioclase
possesses maximum
ex-
tinction
angles
of 24
degrees
in
sections
at
right
angles
to
the
albite
lamellae and frequently
exhibits
zonal growth.
Biotite,
largely
altered
to
chlorite, magnetite,
apatite and
titanite,
are
the
principal
accessory
minerals.
The San Lorenzo
batholith has produced pronoimced
contact meta-
morphic changes in the tuffs of the older series in many places where
it
has come in contact with them.
In some instances,
all traces of original
structures have been
obliterated with
their
conversion
into hornblende
schists.
These
changes
are
further
discussed under
contact metamor-
phism.
The fact that the San Lorenzo quartz
diorite
is a younger
in-
trusive
than
the Juncos
gabbro
is
clearly shown
northeast
of Jimcos
where
small tongues
of quartz
diorite, which
are part
of
the Luquillo
stock, project into the gabbro along its contact with the latter.
Yahiicoa
granite.—Around the borders
of the San Lorenzo batholith
and frequently in contact with
it, there are a nimiber of small stocks of
coarse granitoid, frequently almost pegmatitic granite, which will here-
after be designated
as the Yabucoa granite on account of the excellent
exposures that may be seen in the vicinity of Yabucoa Point.
The largest
outcrop, which
includes
the above exposures and which covers an
area
roughly 4 miles long and
li/j miles wide, occurs along the coast between
the Yabucoa and
Candelero
playas.
Other
localities
along
the
south
coast where the granite is exposed are Point Yeguas, Cape Mala Pascua,
and Patillas Landing.
In the interior the only occurrence observed
lies
about
1 mile northeast
of Las Piedras
in the form
of a
strip
14
of a
mile wide and 2 miles long adjacent to the San Lorenzo batholith.
The Yabucoa granite
possesses
a very
coarse
granitoid,
in some
in-
stances almost pegmatitic texture.
Under the microscope, the two chief
constituents are found to be orthoclase and quartz, the former sometimes
being
light pink
in
color.
Only minor amounts
of
plagioclase,
rarely
ifio
8viEjriF/r
,^'/7i»r/-:r of porto mico
showiji*!" maximimi
e.\tiiii'tioii
aiigics
gri'ater than
J,1 dewTees
in
sections
at
ri<i-lit
iingie^
to the
albite
laiiii'lk'. and
a
littlf
hiotiU' mvum m
it.
Hornblende
is; pnietienlly
alxsent.
Othi>r aci'e<sory miiierals are
apatito,
inapiotite. Hiid zireon^.
A thhi
ssfctioii of th«*
gTaniti:' from
tlie vieioity of Yal)ueua Playa
cuii-
sists
chietly
of
orrho(;hiHe and
qmirfz
in
ali<>trio)nor|)hic
inforgrowth.s.
Its aeeessory eniistitnents are
[dagioelaHe. dark
In'ouii
hiotilf.
ma^jiictite,
and
a]:nitite.
Figure
3»i
represents
a
photomicrograph
of
tln^
granit<=.
In the vicinity of FraiU» Point
tlie granite
still has a very mueJj
siinilar
oompo.sition.
A
little
zireon^ appears
iji
sections
of
t!ie
rock from
this
locality.
At
I'atiUas Landing a mierographie intergrowth
of orthoda^?
a.nd quartz
is a [)roininent feature
in
the
roc-lv' which was nut noted else-
wJiere.
The
coHrHe,
aliriost
pegmatitic
Yabueoa granite
1
mile
Jiortti-
east
oi" Las
Pieilras consists largely of light pink ortlioelase and
(piartz.
}*lagioelase and
hiotite are
its
cliief a('cessory
<M)iistiriU'nts.
The
strin:-
tiire on
the whole
is
allotriimiorphie.
]!ktierr)perthitic
intergrowths
of
FETTKE, OEOLOay OF THE HUMACAG DISTRICT
IGl
orthoclase and
plagioclase
of minor importance
are
noticeal)le
in some
of the sections of this rock.
In
the
vicinity
of Yal)ucoa Plava
the
contact hetween
the Yabucoa
granite and
the San Lorenzo
batholith
is
exposed.
The
fact
tliat
the
Yabucoa
oTanite
represents
a
later
intrusion
is
plainly
evident,
as
it
ramifies the quartz diorite
in
all directions in the form
of
dikes.
The
latter has also, in part, been converted into gneiss
in the
vicinity of the
contact.
These metamorphic
effects are described
ill more
detail under
contact metamorphism.
Patillas quartz
monzonife.—A third group
of
stocks
intersecting the
ohler
series
of the Humacao
district
is made up
of granitoid
rocks
of
tlie mineralogical composition of quartz monzonite.
Such stocks are par-
ticularly well developed along the Patillas valley and the name Patillas
quartz monzonite
will,
therefore,
l^e
applied
to
this
tvpe
of
intrusive
hereafter.
Similar
stocks
also occur along
tlie
Llaurel
valley
nortli
of
Arroyo, along the military road southwest of Caguas, along
tlie Tural)o
valley, along the road leading from Cagilas to Juncos, along the contact
between the San Lorenzo
batliolith and the older
series 3 miles
east
of
Juncos, and in the vicinity of N^aguabo.
The areas covered by their out-
crops
are usually
small,
in most
instances being
less than
^,/>
mile
in
diameter.
The
lar(>"est of these intrusive bodies, that
in
tlie vicinity
of
the Patillas reservoir, however,
is nearly 4 miles long and ha^ a maximum
width of about 2 miles.
Tn most instances
plagioclase,
orthnclase, hornblende, and
quartz
are
the main constituents of the
Patillas quartz monzonit'\ while maunetite,
apatite, and
titanite
are
the
principal
accessory
minerals.
Wliile
the
texture
is usually granitoid,
it
is considerably finer
tlian that
develo])ed
in
the San
T orenzo quartz
diorite and Yabucoa
izranite.
Occasionally
alon^T the borders of the larger stocks or throughout
in
tlr^ case of some
of the smaller ones
it
is porphyritic.
The
Patillas quartz monzonite
at the
southeastern
end
of
the
stock,
which
occurs
along
the
Patillas
valley
in
the
vicinity
of
tlu^
Patillas
reservoir,
is
a fine granitoid dark greenish-grav rock
in
wliich
feldspar,
hornblende, and
a
little
quartz
are
visible.
In
thin
section
the
chief
constituents
are
found
to
l)e
orthoclase,
pla^^ioclase,
green
pleochroic
hornblende, and
quartz.
^Maii'iietite
is an
abundant
accessory mineral.
Tlie feldspar
is cloudy from
tlie development
of
kaolinite and
sericite.
The hornblende has altered
in part to epidote and
chlorit^.
Along the
southern margins
of the
stock the texture
is
porphyritic.
Phenocrysts
ju^edominate over groundmass.
They consist chiefly of
plairioclase with
maximum extinction angles of IS degrees in sections
at rigbt angles to
1(32
SCIENTIFIC SURVEY OF PORTO RICO
the
albite
laniell^e, indicating andesine,
a ferromagnesian mineral now
complete!}^ altered to epidote and
chlorite, and a
little
orthoclase.
The
gronndmass
consists
of an
allotriomorphic
intergrowth
of
plagioclase,
orthoclase, and
quartz.
Micrographic intergrowths
of quartz and
feld-
spar are common.
Considerable sericite has developed from the feldspar.
At the northeastern end
of
the
Patillas
reservoir
the rock
possesses
a
fine
granitoid
texture.
Plagioclase,
ortlioclase,
augite
frequently
sur-
rounded by rims of green hornblende^ and quartz in part in micrographic
intergrowth with
orthoclase^,
are the principal
constituents.
Magnetite
in
abundance
and
apatite
are
the
accessory
minerals.
Considerable
sericite and kaolinite as well as some
chlorite have developed from the
feldspar by
alteration.
The
augite and
hornblende have
also
in
part
been altered to chlorite and epidote.
A small stock of the Patillas quartz monzonite on the west side of the
Llaunel valley along the playa border consists of a
fine granitoid gray
rock
in
which
feldspar,
hornblende,
and
quartz
are
likewise
visible.
Under
the
microscope,
plagioclase,
orthoclase,
quartz,
and
pale
green
hornblende are seen to be the important constituents.
Almost
colorless
cores
of augite surrounded by
pale green hornblende
are
also
present.
The quartz occurs in part as micrographic intergrowths with the ortho-
clase.
Magnetite in abundance and apatite occur as accessory minerals.
Some
kaolinite and
a
little
sericite have
developed from the
feldspar.
The hornblende has partially altered to chlorite and epidote.
The two small intrusive masses of Patillas quartz monzonite occurring
3 miles east of Juncos along the contact between the San Lorenzo batho-
lith and the older
series possess a porphyritic texture with phenocrysts
in
excess.
The predominating minerals are
plagioclase,
orthoclase, and
a dark greenish-brown hornblende.
The groundmass
is made up largely
of feldspar and quartz occurring in part
as micrographic intergrowths.
Magnetite, apatite, and titanite are the principal accessory constituents.
Abundant
sericite
has
developed
from
feldspar.
Small
amounts
of
chlorite and epidote also occur in the rock as alteration products.
The other areas of
Patillas quartz monzonite, shown on the geologic
map, are very similar in texture and mineralogical composition to those
described.
The
relationship
of
the
Patillas
quartz
monzonite
to
the
Yabucoa
granite is shown along the coast, 2 miles southeast of Patillas where their
contact
is exposed,
as shown
in figure
34.
The rock
at the
left
is the
quartz monzonite and that
at the right the granite.
The point of the
hammer rests upon the contact.
The quartz monzonite 15 feet from the
contact possesses a fine granitoid texture.
Orthoclase, plagioclase, augite,
FHTTKf], ilKOLOdY
(JF THH HUMAVAff D/h'TIf/Cf
and
quartz
are
its
inain
C'onstitiunits.
8i>iTie
of
tlie
([oartz
occurs
in
iiiicrograjiliic iiitergrowth with tlie orthoclase.
A
little green hornblende
is
prt'sent
in
the
ftirni
of reaction
rims around
the
aogite.
Magnetite,
a|)atite, and
tiititito an; acjcesaory minerals.
Sericite has dev(!loped from
' felds^iinr,
<,:Iilorite from augite, and a
little eoidote occurs along minute
fractnres
in
tln^ r<M'k.
Approaching tlie con i.act tlie t.cxture becomes
fiiicr
and
liner until at
tlie eontiict it is felsitic and the rock beeonies a
quarti!;
hitite.
The
intrusion of the quartz monwnite
app<!ars
to haw
liad. no
eii'eet upon thc» I'ahueoa granite, whirh is clearly the older rot-k.
DIKES
Dikes
of
iguenus
rocks,
cutting not
only
the
older
«) the major
intrusives
in
it,
are
proiniiicjit
geologic
ries
itself hut
;»atures
in
the
Hnnuicau
district.
They range
in thJekTiess Ironi a
fraction of an
inch
op
tn 55
iVet and perliaps more in some instances.
On account of their
great
alnoidance and the small thickness of -many
(d'
them.,
jh) attempt
has been made
to
sliow them upon
t.h(! geologic map.
Vw
pu.rp()ses of
description,
thev
can
he
divided
into
two
types,
namelv,
tliose
of
a
siliceous characti'r and those of intermediate to basic composition.
The
major period
of dike formation appears
to have commencerl
soon
after
tlie
intru.'iioji
of
tlie Han
Lorenzo
bath<jlith and continued
until
shortly
after the appearan<'e of the
Patillas quartz monzonite.
S'ilix'fMUH
dikes.—The
siliceous dikes consist
of
granite,
granite peg-
matite,
aplitc, granite and
rhyolite porphyry, and
rhyolite.
These
are
164
HVIhlNTlFia 8Ull¥KY OF PORTO RIVO
tiiidoubtedly eloselv related to the Yabucoa granite an<l
tlie
tivtit two are
probably jdentieal witb
it.
'rbey were found cutting oidy tbe Ban
Lo-
renzo quartz
diorite.
Tbe
aplitos and granite
an^d
rbyolite
|)orpliyries,
on the otlier band, oeeur not only
in
the San Lorenzo «piartz diorite but
are also intrusive in
the Yabueoa granite
itself, and,
in one instance, an*
apJite dike was foiuid eotting a quartz mouzoiiite porphyry that
Ih prob-
ably tbe e([uivalent of the i^atilbis.
Granite ami
<ji anile pegnmtUe
dikes.— In
the
vieiiiity
ot'
Quebrarbi
Honda Point,
tbe San Lorenzo (pjartz diorite
is cut by a dike of granite
1 foot 10 inches wide that splits as shown in figure
-lo.
The texture
is
granitoid and
the
injportant mineral constituents are
ortbtH-lase.
iriiiiro-
cline,
(piartz, and
plagioelasc of
the
albite
variety, showing
ni:ixinimn
extinction angles of 13% degress in sections at right ang
hunella:'.
Small amenjiits of
biotite and
a
very
Httle
n,ni
and
zircon^
are
accessory
nunerals.
This
rock
is
undtiu
with the Yabiieoa granJl(\ a stock of wliieh may
be seen
south abjng tbe eoasi.
Pegmatite (hkes cut
tlie San Lorenzo batholith
at
a
ui
.along
its
Ixirders.
Such, a dike,
I
foot
in
thickness,
nia
the Juncos-Saii
Lorenzo road about 4/5 of a mile south
Orthochise in crystals up to 4 centimeters
in
length, and
are
its chief
i-onstitu,ents,
l)ut some quartz and
biotite
a
It resemhh'S
rather closely some of the coarser
pliases
i
granite and
is thought, therefore, to be identical with,
it.
he albite
.patite.
id entical
(nid
feet
of places
en along
• J'nn(-os.
agiotdase
l> resent.
Y abucoa
FETTKE, UEOLOCY OF TEE HEIIAXliO DIBTMICT
Apliie
cH/i'fts'.—ApJite
dikes
are
of
mucli
more
freiiuent
oceiirreuee
thai)
the pegmatite
varieties
aiir!
are not
eoiiliiiecl
to the Ban
Loreiizu
quartz diorite areas
but
also
oeeiir
in
the
YaI)iieoa granite, and
in one
place
along
the
military
road
southwest
of
(^'agnas
in
tlie
viein,ity
tif
K. 41, a 12-foot dike of
tliis type
eutsi a quartz moiizonite p«)rphyry that
has been
ecsrreiatcd
witi)
the
Patilla^s.
Figure
o(»
sliows
a
niuriher
of
aplite
dikes
eiitting
the
Yahueoa
granite
in^
thi^
vieinitj
of
Yahucoa
Point.
The majority of these dikes
do not
ex{;eed
1
foot
in
tliick:ncs.s
and
nsiially
tliey
are
eonKiderably
ill inner,
lliey
always
possess
a
rather
fine
granitoid
iextnre
and
are composed
j»rim;i pally
of
otlio-
elasc and quartz.
Phigiochise, bio-
tite,
muHCovite, and
nuigrietite
are
only
very minor
constitncnts.
frinnHi'-
imd
rhyoUte
porphyrij
ilikf'S.—The
granite
and
rhyolite
porpli\i'y dikes are
tlie most;
aiinii-
dant and important of the silieeous
group.
'J'he
two
differ
from
one
another
onli^ in
tln^
relative
al)un-
dau(;e of tlie phenocrysts present
in
them
and
the
texture
of
tlieir
gronndmasses.
In the former phe-
nocrysts predominate, while
in the
latter
tin:' grcmndmass
Is
in
excess.
(»ranlte
and
rhyt)]ite
porphyry
dikes are rather widely distributed
throughout the Humacao district.
They eut the formations of the ohh-r
series as well as the San Lorenzo (pnirtz diorite ami the yahn<'(ia granite,
but were nowhere found iuterseetiog tlie Patillas quartz umnzonite.
It is
probable,
thtsrefore, that they
fotlowed
closely after the
intrusion of the
former and
before
tln^ appesiranee of the
latter.
The granite and rhyolite porphyries ean
tie studied
to good
yilvajilago
in
the vicinity
of Yabueoa Point where
their
intrusive
relaliou
to the
Yahucoa granite
is
well shown.
The dikes of
this
locality consist
of a
liglitgray coarse i>or])hyritic rock
in which, abundant euhedral as well as
anhedral crystals
of both orthoelase and
<|uartz and oeeasional flakes of
biotite are visibh^;.
The phenocrysts of orthoelase fre(pn:«ntlv reach lengths
H5a
SClEXTIFiC PURVEY' OF I'ORTO RfVO
of 2% and diameters of i% eentiiiieters.
Tliey eoiisi^^t of simple crystals
boiiJided
I'y a
prip]ii and
basal and
(•liiHH)i]uu-ojds,
Carlsbad
twin!:* with
the same
foriiiH
dt'vylopod
on
tbcni may
also
be
seen.
TLie
eidiedral
crystals of (jnartz sometimes reach lengths of 2 eentimelers and diameters
of
1.2.
'Fliey
eoiisist of <-ombiinitions
of
the
hexag'onal
prism
with
the
positive and
negative
rliombohedrons,
t)ne
instanee was
jmted
where
two
crystals occurred
in
parallel growth
in
the
form known
as scepter
'w/ritiih
fif rhiinUl€
P'trphpi'n
cutlintf
l^ati
Lot
the
ririiiitij
of gnrhniiln Hoiflii PninI
qnartz.
Wliile
iiumer(His
plienocrysts
(tf
ciiiartz
arc;
enhedral,
the
nni-
jority are rounded
at
lea.st
at
tire
t-orners.
j^liotite
is not a
etnispicuous^
eoustitucnt.
()ligoela«e, magnetite, titanite. and zircon are minor acces-
sories-
The
gronndnirtss
consists
of
a
fine
niosHie
of
ortlioclasc
ami
quartz.
A
little
chlorite and epidotc
Inivc
developed
from
the
biotite
and serieite from the feldspar.
A ])liot(nnicrograph of the rhyolite porphyry from
a dike
3(1 feet thick
eutting the San Lorenzo qnartz diorite in the vicinity of (^nelirada Honda
FETTKE, GEOLOGY OF THE HUMACAO DISTRICT
167
Point
is shown in figure
37.
It possesses a coarse pornhyritic
texture.
The phenocrj'Sts consist of euhedral and anhedral orthoclase and quartz.
The orthoclase occurs in simple crystals and as Carlsbad twins.
Oliogo-
clase and minute flakes of biotite are accessory constituents.
The latter
indicate a fiowage structure by their parallel alignment around the hirger
phenocrysts.
The groundmass
is cryptocrystalline.
A
little sericite has
developed from the feldspar and chlorite and epidote from the
biotite.
Several dikes
of rhyolite porphyry, cutting a
felsite, may
l)e
seen
in
a quarry for road rock ak)ng the west bank of the Patillas Eiver where
the road from Patillas to Arroyo crosses it.
The phenocrysts consist prin-
cipally of orthoclase and quartz but the rock
is particularly exceptional
in
that
occasional
small trapezohedrons
of
garnet up
to
1
millimeter
in diameter occur
in
it.
Sometimes the garnets appear in the ground-
mass but often they are present
as
inclusions
in the orthoclase.
Occa-
sional plagioclase phenocrysts may
also be
seen.
The groundmass con-
sists
of
a
fine
allotriomorphic
intergrow^th
of
orthoclase
and
(juartz.
Sericite in abuiulance has developed from the feldspar.
Bhyolite.—About 14
i^^^l^
"^^P ^^^^ Quebradillas valley from the military
road southw^est of Caguas, a dike of rhyolite about 10
feet
tliick occurs
in
the rocks
of
the older
series.
In thin
section ouly occasional small
phenocrysts
of
orthoclase
are
visilrle.
The groundmass. which
consists
largely of orthoclase and quartz,
is extremely
tine.
A
little epidote and
chlorite are present in
it.
Dikes of intermediate and htmc com position.—The dikes of this group
range in mineralogical composition from
latites and monzonites on
tlie
one hand to augite and andesites and diabase on the other, and
])ossess
textures
ranging
from
felsitic
through
porphyritic
to
granitoid,
the
felsitic and porphyritic types being the more common.
They are encoun-
tered most frequently in the older
series, but are
also abundant
in
tlie
marginal
portions
of
the
San
I^orenzo
batholith and
are
occasionally
present in the Yabucoa granite and
Patillas
quartz
monzonite.
While
some of these dikes may be older thau
the San Lorenzo quartz
diorite,
it
is believed that the majority followed
shortly
after
the intrusion
of
that rock.
Latite and
nionzonite dikes.—jyikes
of the mineralogical composition
of
latite and monzonite
wTre found
cutting
the
older
series,
the
San
Lorenzo quartz diorite, and the Yabucoa granite.
A latite dike about 20 feet thick cuts the volcanic breccias of the older
series along the w^est fork of the Llaunel Eiver.
IMacroscopically
it
is
a
dark
gray
almost
black
felsitic
rock.
I'nder
the
microscope
a
few
phenocrysts
of
plagioclase and
orthoclase may
be
ol)served.
:Magnetite
168
SCIENTIFIC SURVEY OF PORTO RICO
occurs as an accessory constituent.
The groundmass
is composed largely
of minute interlocking feldspar crystals and a little augite.
Southeast
of Naguabo
Playa^
latite
and
monzonite
porphyries
are
present amongst the numerous dikes that cut the andesitic rocks of the
older series in that vicinity. A thin section from a latite porphyry dike
only 11/2 inches wide contains numerous phenocrysts of plagioclase, ortho-
clase, and hornblende in an extremely fine crypto-crystalline groundmass.
Biotite,
quartz, and magnetite
are
accessory
constituents.
Flowage
is
indicated by the parallel alignment of the phenocrysts, particularly the
smaller ones,
x^ear by, a thicker dike of monzonite porphyry, with pheno-
crysts predominating
over
groundmass,
has
almost
an
identical
min-
eralogical composition.
In the vicinity of Quebrada Honda Point, a quartz monzonite porphyry
dike only 8 inches wide cuts the San Lorenzo quartz diorite.
Plagioclase
of the andesine variety, orthoclase, microcline, quartz, and biotite are
its
prominent constituents.
Magnetite,
apatite and
titanite occur
in
it
as
accessory minerals.
Along the coast near Patillas Landing, a dike
of
latite porphyry, 20
feet thick, cuts the Yabucoa granite.
The prenocrysts consist of ortho-
clase and plagioclase
of the
oligoclase variety:
The groundmass shows
a
spherulitic
structure.
Apatite
is
present
as
an
accessory
mineral.
Chlorite, sericite, and epidote occur as alteration products.
Da cite and quartz diorite
dilces.—Dacite and quartz diorite dikes are
not very abundant.
Along the
coast west of Kaguabo Playa,
a
dacite
dike, 12 feet thick, intersects a felsite belonging to* the older series.
In
thin
section, the rock shows
a
felsitic
texture.
Plagioclase and quartz
can
be
identified
in
it.
Chlorite,
calcite, and
epidote
are
present
as
alteration products.
At
one
place
along
the
Guayanes
valley,
a
dike
of
quartz
diorite
porphyry, 3 feet thick, w^as found cutting the San Lorenzo quartz diorite.
Labradorite, quartz, hornblende, and biotite are
its predominating min-
erals.
Orthoclase,
a
little
magnetite,
and
apatite
occur
as
accessory
constituents.
Andesite and
diorite
dikes.—Dikes
of
andesitic
composition
are
the
ones most frequently encountered in the Humacao
district.
Hornblende
and augite varieties are about equally abundant.
They are mostly con-
fined to the older series and the San Lorenzo quartz diorite, but instances
were also noted where augite andesites cut the Yabucoa granite and the
Patillas
quartz
monzonite.
In
texture
they
range
from
felsitic
to
porphyritic.
A hornblende
andesite porphyry with very prominent
large
crystals
FETTKE, GEOLOGY OF THE HUMAGAO DISTRICT
169
of hornblende cuts the older series near the crest of the Sierra de Cayey
at
the head
of
the west fork
of
the
Patillas
Eiver.
In thin
section^
phenocrysts of greenish-brown hornblende and lesser amounts of plagio-
clase are prominent.
Magnetite
is present as a minor
accessory.
The
groundmass
consists
principally
of
plagioclase and hornblende.
Some
epidote has developed
as an alteration product from the hornblende.
Along the coast in the vicinity of Naguabo Playa, a hornblende ande-
site dike, 3
feet thick, cuts andesitic rocks belonging to the older
series.
Its texture
is
felsitic and under the microscope the structure
is seen to
be diabasic, but bluish-green hornblende containing inclusions of biotite
occupies the space between the plagioclase laths instead of augite.
In the vicinity of Quebrada Honda Point, a felsitic hornblende andesite
dike, 2 feet thick, cuts the San Lorenzo quartz diorite.
A greenish-brown
pleochroic hornblende and plagioclase
are
its
chief
constituents.
Mag-
netite
is abundant.
An occasional grain of augite and a
little titanite
are also present.
A flowage structure
is indicated by the parallel align-
ment of the amphibole crystals.
A 55-foot dike of augite andesite intersects the volcanic rocks
of the
older series along the Llaunel Kiver a short distance below the forks.
It
is a dark
gray, almost black porphyritic rock
in which phenocrysts
of
plagioclase and
augite
are
visible.
In
thin
section,
plagioclase pheno-
crysts of the labradorite variety are seen to predominate but pale brown-
ish-green
non-pleochroic
crystals
of
augite
are
also
prorninent.
The
groundmass contains abundant microlites
of
plagioclase.
A
little mag-
netite
is
also present.
Small amounts
of
sericite have developed from
plagioclase and chlorite and calcite from augite.
Near the
northeastern
end
of
the
Patillas
reservoir,
a
felsitic
dike
of
augite
andesite,
15
feet wide,
cuts the San Lorenzo
quartz
diorite.
Its chief constituents are plagioclase and augite.
A
little magnetite
is
also present.
Some calcite, chlorite, and epidote have developed by altera-
tion from the plagioclase.
Along
the
coast
not
far from
Patillas Landing,
a
15-foot
dike
of
augite andesite cuts the Yabucoa granite.
It
is
felsitic in texture and
plagioclase and augite are
its main components.
Magnetite occurs as a
minor constituent.
Some chlorite
is present as an alteration product.
On
the
point which
projects
into
the
Patillas
reservoir
at
the
east
end of the dam, a dike of augite andesite cuts the Patillas quartz mon-
zonite.
It
is a felsitic dark gray, almost black rock.
Under the micro-
scope,
a few
phenocrysts
of labradorite
felspar and
augite
are
visible.
The groundmass
is made up largely of feldspar laths with augite inter-
170
SCIENTIFIC SURVEY OF PORTO RICO
spersed between.
Abundant magnetite also occurs in
it.
A little chlorite
associated with augite and calcite are pr^ent as alteration products.
A
diorite porphyry
dike,
2
feet
thick,
cuts
the San Lorenzo
quartz
diorite in the vicinity of Quebrada Honda Point, as shown in figure 38.
It
consists
chiefly
of labradorite feldspar and hornblende.
Orthoclase,
magnetite,
titanite,
and
apatite
are minor
accessory
constituents.
A
little epidote has developed by alteration from the plagioclase.
D'mhase
dikes.—Only two occurrences of diabase dikes were noted in
the Humacao
district, in both of which the diabase cuts Patillas quartz
monzonite stocks in the Llaunel valley region.
The diabase ranges from
fine granitoid to porphyritic in texture.
Labradorite feldspar, frequently
zonal, and a pale green non-pleochroic augite are
its chief constituents,
the
latter
filling the
space between the feldspar
laths.
Magnetite and
highly pleochroic brown biotite are the accessory minerals.
CoxTxVCT Metamorphism
Contact metamorphic plienomena in the older series are confined ])rin-
cipally
to
the immediate
vicinity
of
the San Lorenzo
batholith.
Tlie
metamorphism has
resulted
largely
in
the
conversion
of
the
tuffs and
associated andesites of the former into schists and gneisses,
cliiefly of a
hornblendic variety.
This has been brought about by a recrystallization
of
the
original
constituents
of
the
former and
the
development
of
^
foliated structure
in the
resulting
rock.
Apparently,
no new material
has been added from the outside.
The pressure exerted
])y the invading
magma upon
its w^alls as
it was making room for
itself appears
to have
been
responsible
for
the
development
of
the
foliated
structure.
The
schistosity everywhere is parallel to the contact.
The conversion of portions of the Collores limestone into an aggregate
of lime iron silicates is also believed to have been the result of the intru-
sion of the quartz
diorite magma of the San Lorenzo batholith, but in
this
case emanations from the magma
itself
reacted with
the calcium
carbonate of the limestone to produce the metamorphic minerals.
In the vicinity of Yabucoa Playa, the intrusion of the Yabucoa granite
has converted the San Lorenzo quartz
diorite
into a gneiss just
as the
invasion of the latter converted the tuffs and andesites into
scliists.
HORNBLENDE SCHISTS AND ASSOCIATED ROCKS
The Maunabo Eiver, along
a considerable portion
of
its
course,
fol-
lows rather
closely the contact of the San Lorenzo quartz
diorite with
the
tuffs of the older
series and the
effects of
this
intrusive upon
the
FHTTKtL (lEOLOar OP THE HVMACAO lHHTh'li'T
latter cau, therefore,
lie
.<tiulie'l
to g'ood
{iclvaiitagx* at WMmy places
its course.
Ont;ero]>K of iiietHinoi'pliie rock appear about SU. m\\v<
the town of
MciiiiialM) ami
eoiitimie at
intervals for
Jienrlv o
niileH
mil up
HtroHin the imi
Oulei'ops
eiieoiinteri>(l
jhk.h'ss
h, dark ^niy color
and line erystalline texture.
The foliated ntrurture
is imi
very iiotieoahlo
to the oakwl
cje.
Under the
iiuVroHi'<»pe, the
ruck
\< nvn
t<»
crjiisisf
of
on
iiiterloekino-
luosair of
au^u'ilt'. orthoelaso and
a
littic
oiiartz.
.Small
172
l-<CiENT/FIV 8UHVMY OF POHTO MICO
grains
of HuigDetho are
lairly
abiiiulam
arid
a
Jittlc
niiisecnjte
is
also
])re!*eiit.
The
<iiig'jtf
is a pale brown ish-greeii iioii-pleochroic variety full
of imiiisions of oilhoelase hiuI nuaj'tz as well
as magnetite.
Often con-
siderable patches of
individual
aiigitc grains
heliave as single nnits op-
tieall}-.
A slight indication of a foliated stnictnre may he ohsemtsd, due
to
the
greater
eonceiitration^
of
aiigite
grains along
certain
bands and
their parallel orientation.
A photomicrograph of
this pyroxeini schist
is
showji
in
tigiirc
d!>.
A
little further upstream plagioehise and
a
little
Fig.
30.
I'lioti
Orth
liornhlemlc
also appear in the
schist.
About a mile above
the
first
lo-
eality,
typical hornblende 'sctdsts make
their
appearance.
TJiese
also
possess a
tine to medium crystalling texture and show a slightly foliated
strnetnj'e dne
to
rfie more or
less parallel alignment oi the liornbleiide
gi'aijis.
In tldn seid'on, the ro(;k is foimd to consist of a mosa.ic of horn-
blende,
plagictelase.
orthoclase, ami
(piartz
gj'ains.
Sume magnetite
is
also present.
"TIjc h<rrnl)]ende
is a deep green, highly pleocliroie variety.
Compared
with
tlie
aiigite
grains
of
the
pyroxene
schists
it
is
rela-
FETTh'E, OEOUMjy OF THE UrMAVAO iJhSTRIET
ViS
lhx-]y
free from
liielusioiis.
Oecasioiwl
larger gvalm of orthoclase
ajitl
plagioclaHt! with
ragge,] hmkn aim oeeur
in
t\w mek.
i:iiere was en-
(•ouJit;cretl
a
finu
grairioil
<|uart2ite
214
.mile.s
above
the
first
loeality.
Tlris
tiHjsists proH-ij)uriy ot
y
fine jnonaie
of: quartz
wifli only
oeeaj«iorjal
grains id ortlioelase ami plagioclase.
Now m\d then a ehister
ut' pxromne
grams ma.v also be oh.servcd in
it.
In, palehes
elilcu'itc and soricite oecur
iH'twern
the
iiiuirtz
m-ainH.
t
t
I
t
I
I
g mm.
Fi(i.
40.
PhuitMlvrMiftiph of hunhlciulr
M-hi.si fmm^ Tuim
Point
in
tlio vicinity of Tmui
Poijit, sontlieast of Afaunabo, a small area of
Inimblende
schist
an<l
associated
ijuartzite
is
exposetJ abmg
ihv.
coast.
The liornblende schist possesses an extnynely
line crystalline texture
ion]
a
pronounced
foliated,
strmd^iire due
to
the
concentration
of
the
hor,ii-
blende in hands l)etveen tlie Cfuartz grains.
The rock crKUsists principally
of hornblc!nde and
quartz, the former
lu'lng
.relatively
free from
inclu-
sions
as
arc
tin-
quartz
grains.
A
little
ortlHiclase showing
Carlsbad
twin],ii,ug
an<l
plagiocksc
oxhibitiug
albite
twinning
are
also
present.
Pyrite and .magnetite
in
little grains
a]«o oecnr in
ttie
rock.
,A photo-
micrograph of this schist
is sliown in figure
-P).
The quartzite associated
:i :
i
SSClHXTfl'H' Hlli'VHV OF PORTO UK'
with
it
is
iui
t'xtrt'iiifly
lim-u-raimMl
\arit»ty,
a
pliutoHiicrograidi
of wiiich
appoars
in
ih^wrv 41.
It
cuJisists of
;i
fine musaic oi jiTrtimlMr qiiyrt/
in
which
a
liftll- hornhle.idc
in
piiraHd
aligiiiiifiit
nirtv
ho ohsprvcd.
Soiti.-
iHa.yiii'tit!'
,ii-?'ain8 arc also prosont,
^Icraincrplioso.l
tiilfs
also appear
at
the
headwrttin's
of
tin'
(;iiayjiiH'>.
Hivcr
and
alou,^
the
divide
hctwccn
that
stream and
Uh'
Did
I'^spin-.
hhvcr.
A
dark
.ya-ay
line
(awstallinc
spctdnieii
from
iho
hitter
l..cnlii>
I'xdiihitiim- a (h'stinetlv
roliahnl
striudiire
wa^^ iunnd under the
niiernsrup,.
ii|.
ed'
alternate
Imnds
uf
aimate.
feld.spar and
and
lu.rnhhaide.
The au-ite
is nearly e<dnrle<.<
i„
id. in
see! ion.
Consaha-ahle
(tatehes of
it- unnm hehave optieally a^ sin,u'le ynits.
The h.^Tlan- cidored
hand.^
eonnist
]>rincipally
oi:
pla.^lu(dase.
urthoehi.M..
and
rpjartz.
Th.-
hornldemle
is
a dark hrowninh-ea-een^ j.leeehmie
variety.
While
its graJiis
are
al.Mi
in more or
less
parallel alieannent
alun-- eertain
hands,
patches
of them do
lint
behavi.
opti<-ally
as units when
viewed helween
crossed
nie<ds.
Oeeasional
hiru'er
rag.a^d
crystal^
<d*
aiie-jre
full
ol
mae-neiip.
fi-:tti\h. i;Enijj(iY
<n- the ih\\ia«\\o ihstuivt
\]~^
inrlusioiis
iiiul
liaviiig
1ionil>h>ii(|(.
<;it.vt.l<.|K-<l
iilonu- their
iK.rdtTs, as
well
nK
ii!
their
iiilfiiors,
ar«'
tilsu
pn'seiil.
'We^i-
iiirtv
represeiii
jui.uitr
.•rystulH of
liic (rniium]
tut! frum
wl-jidi
tlu' schist has
Iteeii
.leriwd.
'
.Alojiu;
liu'
l'ii<>;iirt.--8an Lnreiizo
yivm]
lu.rtliwest of
Sais J.uTenzo,
in
the
vi<-inity of
K.
8,
rhe ineiiriijorpliit:
hell
is rtpproxiiiiatelv
<JO0O
iVet
wi<le.
'Mil' metijmorpJios.-d
tuff still eoiitaiiis oriuma]
rrt«rjiXM! and eIou<ly
«'rystals
of
pladuelase
luxl
nrth<M-his,'.
Thr
recryslrtllize.l' portions
cunsist
uf
n
mosaic
of
green
pleuchrnic
lioriiitlctHlc,
pla.i.-i«rlase,
orthudrtsc,
and
• piartz.
Tlic
latter feldspars arc dear.
A speeijne]] of liornhlcnde schist e.lttained
-i nnles sciutlnvest of .hnieus.
ill
tlie vit-initv of the San
f.oren/u ipiartz diorite conta<-t. contains
fairlv
nimiercniH
lar-X'
ragged
crystals
of
pl«gio<-ht-^i..
chuided
hy
the
deveh.p-
rnent of
sorieite
in
ilienu imhcdded
in
a
rccrvstallizcd matrix composed
largelv
(d* dark
hluish-green
In.rnldcndc and
.^iiiaDer annnyit>:
..f
plagiu-
ela«%"qnarte. and
possihlv orthcchise.
The
foliated
stru.'tnre
is
dvie
to
the mniUvl alignm<mt of 'the h<n-nhlendc <.rystals ah.ng their long axes.
176
SCIENTIFIC SURVEY OF PORTO RICO
A narrow belt of schists l)or(lers the San Lorenzo quartz diorite along
the soiitli side of the Huniacao Playa.
A photomicrograph of a specimen
obtained from a small hill whicli projects above the playa level south of
Huniacao,
is shown in figure 42.
It
is a dark gray fuie-grained foliated
rock consisting of bluish-greeii pleochroic hornblende, quartz, plagioclase,
and a little pale green non-pleochroic augite.
The latter sometimes has
rims of hornblende around
it, particularly in the case of the larger crys-
tals.
The schistose structure
is due chiefly
to
tlie parallel alignment of
the longer axes of the hornblende.
Inclusions of metamorphosed
tuff and andesite are of rather common
occurrence
in the marginal portions
of the San Lorenzo quartz
diorite.
They
vary
in
diameter from
a few
inches up
to many
feet
in some
instances.
M ETAMO RPirOS J:D L 1MESTOXE
The
Collores
limestone, whose
outcrop
occurs
along
a
belt
trending
S. 53°
E.
for a distance of nearly six miles about
3/4
^^^^^^^ northeast of
the San Lorenzo batholith, has been recrystallized into a gray to white
crystalline rock of medium texture, in which
all fossil remains that may
have occurred
in
it originally liave 1)een obliterated.
In places, this lime-
stone bas be(4i converted into an aggregate of lime iron
silicates by con-
tact metamor])hism.
It
is with these metamorphosed portions tbat the
magnetite deposits of eastern Porto Rico are associated.
Contact metamorphic phenomena
in
the
Collores
limestone
can
Ije
studied to good advantage in the vicinity of the third magnetite
de]iosit
southeast of Juncos in the barrio of Collores de Piedras.
Northwest of
this magnetite
deposit,
Avhicli occurs very near the top
of
a northwest-
southeast
ridge, prominent ledges
of the limestone crop out
that range
in mineral
composition
all
the way from
crystalline
calcite
associated
with only small amounts of
silicates
to those
in which
silicates
are the
predominant
constituents present.
A
thin
sectio]i
of
the
least
altered
rock consists mostly of calcite and a little ganiet intersected by fractures
along wbich quartz, specular hematite, and possibly magnetite have been
introduced, replacing the calcite.
This was followed by the development
of chlorite along fractures and between calcite grains, replacing the latter
in part.
Tlie
final stage was the depositioii
of
later
calcite in the form
of minute veinlets cutting the other minerals.
In the more altered rock,
a photomicrograpli of which
is shown
in figure 43, garnet, showing pro-
nounced
optical anomalies,
is
tbe predominant
mineral, having almost
completely replaced the origiiuil calcite of the limestone.
Magnetite and
specular hematite liave
1)een introduced along fractures, accompanied bv
the development
of
chlorite.
Later,
veinlets
of
calcite and quartz
cut
FKTTKE, i.;tl(HJK:y OF THE
lll\UA('A() DhsTUIVT
;n';
the
other
Jnirlerrtl^^.
'V\w
liiiirn-^toiie
i.s
cvcrlaiii
by
an
aixh^^ite
at xhU
locality.
Only tho upper
purti<in
dirpetly
iiiulerneatli
tlic
aiulesit.o
has
Ik3C'ii
olf<'ct»:Ml, as rthovo
«1e>.'ribf.<-L
Toward
tho
soriihoaHt.
tlie
lungiu'titp
ih'ixisit
»-ast
ol:
.fiiiieos
is
^w,^
(ioedcMl
by
a
belt
ol*
niftrtiuorphic
roek
whieli
is
hi'liovecl
to
bav<!
timi
derived
froiri
tiio
(.'ollorcs
l)m»>sto-|)(y
fH-rhii};)s
a
toJTaeeous phase.
Kii-
rneroiis ytromiiieiit
\(H\<te< crop
ont; along the
hillside.
Large wigs oeeur
,,.-^^gwsf:;w^^^^^
m
it
in
].lacts.
A
niirr..sropi.-
.'xainiiiatHHi
of
tie-
m.-k
r,-veals
il,at
it
ermsi.st-
largely
of pymxtme
ati.l
gHrin-t,
the
gariH'f
having
develop.-d
after
the
pn-rnxene.
Cal.-ite,
epidote.
quartz,
ami
talc
an-
present
in
minor
anionnfs.
(Iiloril.'
has
developed
along
nniuite
rra<-tnres.
.\n^
other Hj)eeimpn.
tak.ei from tbo wall^
<.r one of
the
vu-v
..onsisls
birgplv
of aimet
an.l
aniplubole'.
tngetie^-r
willi
sonn-
talo.
Th.'
garn.d:
appar^^
enik formed
first then the ainpbihole, and
linally the
tal,-.
Hornblende
and' pyr.)xene
sehisfcs.
nndouhledly
derived
from
aielesitie
tnlf-.
overlie
and underlie the motaniorpliosed
iiitH^stone.
tlH
"IHMlFfV KVRVEV OF
l-:U{T() RicO
(ianit't
roi-k.
as.-<.cirtt»«(l
with
the Colloivs
liiucstoiic,
ivrtH
iil^o
ol)S<.rvwl
iU om- hviilHy .near
llie
M.-iithoiiHt eiul of
tlic
Iniicstoiie
belt.
QiKiHz
>tiotitr.
,itiriss.^^^^^^An
tlie
vicinity
of
Valiur<tii
l>lay«.
the
Hiiii
Lorenzo
(jiifirtz diorite
lias
heoii eoiiverted
iiitu
a
giiei-s by
1Ih>
ii)fri!si(»i)
of the Yahueoa
u-ranite.
The
mieiss
itself
Is mi
hv
imiiierons
totii.'-ue!>
of the eoarw
Yahiieoii granite and
l)y
aplite
diken.
The
reek
posses.^eri
a
mr'iliiiiu
t-rystalliiie texture and
n green ish-g-ray
<:olor.
It
Hhn\v.<
a
din-
tiiuily
giH.i.<se.id
.-tnietjire.
in
thin
se<-tinu,
it
is seen
(o
r-ensisi
prinei-
Qtz:
hloc
Orth
•Kft
>!
>,
pally
of
])higieelas(\,
qnartz. some
*)rth(iehise, and
brown ish-greeij
horn-
hh-nde.
::\!a<iiietit<%
titanite,
biotite, now mostly
altered
to
eldorite. and
apatite
ore
tlse
(diief
aeeessory
eonstitiieiits.
Tlje
hornblende
has
been
affected must
Ity the shearing
produeetl
by
tJie
preswnre
exerie<l
hv
the
invading gj'anite magma, but the
qnai'tz
also shows
a
pronomieed
eata-
clastie
strncfore.
The
Mdspar^i
lia\'e been
affected
the
least.
Herieite
and
kaolinite have developed from the
feldspar,
tbe
plagioehise having
been
altered more than
the
ortboclase.
Tlie hornblende
lias
been
emi"
FETTKE, GEOLOGY OP THE HUMACAO DISTRICT
179
verted in part into chlorite, epidote, and
calcite.
A photomicrograph of
this rock is shown in figure 44.
FOLDIISTG
Tlie structure of the formations belonging to the older
series
is com-
])lex.
The uplift which followed their deposition and which was accom-
])anied
by
the
intrusion
of
the
San
Lorenzo
batholith
and numerous
closely related stocks, produced a folded structure in them that
is rather
difhcult
to unravel on account
of the
relative
scarcity
of outcrops and
the massive character and weatliered condition
of a large percentage
of
those that do exist.
Eeliable strike and dip observations can be obtained
at only rare intervals.
For this reason
it has been possible
to show the
trend of the folds only in a general way on the geologic map.
In the western portion of the
district the folds have a northw^esterly
and southeasterly trend.
Starting at the south, a syncline; followed by
an anticline and another syncline, occur.
Another anticline
starts near
the headwaters
of the Turabo Eiver and trends
slightly
east
of north,
just
east
of
the Turabo
valley.
As
has
already
been
pointed
out
in
connection
with
the
description
of
the
Collores
limestone,
a
syncline
apparently exists northeast of the San Lorenzo batholith, between
it and
the
Luquillo
stock, which
also
trends
in
a
northwesterly-southeasterly
direction.
FAULTING
1^0 direct evidence for the o(^currence of faulting on a major
sc^ale was
found within the limits of the Humacao
district,
althougli
in
sonn^
in-
stances the abrupt change
iu
character
of
the formations
within com-
paratively
short
distances
along
their
general
strike and some
of
the
topographic features of the district would seem to indicate that there
is
a possibility that displacements of considerable magnitude, due to
fault-
ing, may exist.
In the few cases where faults were actually observed
in
outcrops under conditions that made a measurement of tlie displacement
possible it was always found to be small.
ALLUVIAL
D7^:P0S]TS
Unconsolidated
alluvial
deposits
form
tlie
broad
plains,
knowii
as
playas, that occur along the lower courses
of
all the major streams
of
the Humacao
district:
For the most
part,
these have accumulated
in
estuaries formed by the drowning
of the
river valleys, but
in some
in-
stances,
particularly along the south
coast^ they represent
alluvial fans
that have been built out
into
the
sea by the streams
is>^uing from
the
FliTTixi:. CFJHJKIY hF
'111 F. fK^h'ACU) FHSTh'ICT
^•ii»H)<'lH
uiiv
Vn,-H>
ilo|)n.<i!s
hihI
ilum^hx
Inniimu^ Wrnm'^
alon:
valh'VK.
Siirh
disswK'd
alluvirti
dt'iiosits
an,'
soim'tinics
ciu-oi
iiliiiost at
iIh' head
n! a
strcani,
a<
is
tlir-
i-tm>
with
the
(>x|Kisiiri'
ill
fi.mm. d:.
ivliii-li
ui-i-ur^
=i|.u!..-
P,..,.,
|»nl>,v Cn-ok.
i!'
"^
'
^
;' . ^
':
-^"Tf^$^^^4
^If^.
^,
fH
ii-^m^
-#::.
_,,
iKPl.
-w
:
' " -ji^- ''^ :^'^'' iiiil :V
;: »J ' T
.:.,.
' • > '*
"•
1
"'. '
. >:!^
. 3-2»'li
•».
.'HSi'iUiallv
vuui;-^ from
a
uvnl,
'nni-
<;l
til
lluiuarnt.
.Ifstrirl;
mIim'UII.
While
)M,
U,y.<lU
WCIV
l.S-2
^VIEXTIFIC SJ'UVKY OF PORTO
U!('{}
(liseoverod
in
tlie older
series
in
soiitheasteni
l^erto
iiieo
!)y
the
autlior,
suffieieiit evidence has
he(Mi
uathered
hy
othtn-
investigators
in
adjacent
areas
to lead
to the conclusion
that
this series
is hirgely confined
to the
u})])er
Cretaceous,
altlu)U<>ii
tliere
is
a
{)ossil)iIity
that
a
|)ortion
of
the
]o\\"er
])art may be lower
Cretact'oiis
in age and sonu' Eocene sediments
may he included in
its up[)erinost niem!)ers.
During Cretaceous times, Porto Rico was the scene of intense volcanic
acti\ity,
largely
of
the
ex[)losive
type.
Enormous
(juantities
of
frag-
mental
material
were
extruded,
cliiefly
of an
andesiti(^
character,
that
]»uilt u]) the great beds of volcanic
tuffs ami breccias that now comprise
the major portion
of the ohUu;
scries of
tlie Humacao
district.
A por-
tion of the eruptions, liowe\er, was of a quieter ty])e and
resulted
in the
Java
flows,
also
largely
andesitic.
that
are
interbeddtMl
with
the
frag-
mental
rocks.
I^ot
all
of
tlie
latter,
liowever,
readied
tfie
surface,
as
some
are clearly
of an
intrusive
cliaracter, having forced
themselves
as
slieets and sills amongst
tfie layers of tuif and breccia extruded by ])revi-
ous eruptions.
]\luch of this volcanic del)ris, ]>er]ia])s the major portion of
it. un(h)ubt-
edly accumulated under sub-aerial conditions.
Tlie
oecurr<'nce
of
occa-
sional
beds
of
limesto]u\
such
as
the
(N)llores,
interstratihed
with
the
volcam'c
rocks, however,
indicates that
the
district did
]H)t stand above
sea-level
throughout
the
(*retaceous
|)eriod,
but
that
oscillation
with
res|)ect
to
sea-le\el
took
place from time
to
tinu\
which
resulted
in
at
least
j)artial sul)mergence of the area at
iirtervals.
Toward the close of the Cretaceous, a general
uplift took
place which
was accompanied by the
folding of
tlie volcanic and
a^soeiatsMl sedinuui-
tary rocks
tliat comprise the older
series.
Fresh accessions of magmatic
material appeared which
invaded
the Cretaceous formations, and
in
the
Humacao district resulted
in the formation
of the San Lorenzo batholith
and the Luquillo and Caguas stocks.
AMiile
practi(^ally
all
of
the
volcanic^
rocks,
both
massivf^ and
f rag-
mental, are either amlesites or closely related
types, the
later intrusions
sliow
a nmch
wider range
in
chemical and
mineralogical
composition.
Ap])arently, the primary magnuis from which the various igneous rocks
of the llumac^ao district ha\e betvn derived were of an amh^^itic or dioritic
character.
During the earlier stages of igneous activity.
wIkmi cf)mpa ra-
ti vely
ive(' commum'cation
with
the
sivrfacH^
pr(nailed,
s'crv
little
differ-
entiation occurred
in
this magma.
Later on, however,
wIkui new acces-
sions of igneous material
bceanu'
largely of an
intrusive
chara(^ter. such
differentiation
did
take
])lac(>.
As
a
result,
(]uartz
(lio!'it(\s
are followed
by granites ami
quartz nionzonites
in
the form of batholith^ and
stocks.
Firi'TK/j, aiu>L(>(;y of Tin: hi wiacao Dh^TRjcr
iSo
The
(like rocks
aceoiii])aiiyiiig'
this
[Period
oi'
io-neous
activity show
ev(vu
greater
variation, ranging
all the way from
aplites and granite pegma-
tites at the one (extreme to diabase at the other.
The diabase dikes that
are occasionally foun<l cutting
tlie
(jiiartz monzonite stocks
aj)])ear
to be
the last manifestations of igneous activity in
the llumacao
district.
After the uplift which marked the dose of
tlie Cretaceous, there
is no
direct evi(]ence that any (extensive ]^ortions of
tiie ITmnacao district, with
the
(^xception
of
the
i)Uiya
areas,
again
))ccome
snl)merged
below
sea-
level.
Tlie later geological history of this area must, therefore, be
inter-
])reted from
its
j)]]ysiogra])hic develojtment and from what took
])hu'e
in
other portions of the island.
From tho fact
tliat nowliere are
tliere any
igneous
rocks found
associated
witli
the
Tertiary sediments
tliat
occur
ah)ng th(^ north and south coasts outside
tln^ Hunuu'ao district,
it ai^pear's
that
volcaid(^
acti\ity came
to nn end with
the
u])lift
tliat marked
the
close of ih(^ Cretaceous.
During
tlu^ early Tertiary,
(U'osion
^^'ore
a large
])ortion of the lluma-
cao
distric^t down
to a ])eneplain.
It was at this time that the extcmsive
erosion
surface,
rcdcrred to
as the
ui)per
|)enej)lain
in
the
(^irly
])a]*t
of
this re])ort, was developed.
Tins was followed by anotlier uplift of
]Mn'-
ha|)s 1000
f{\A.
Erosion again
set
in and
developed
a
lower
])ene|)lain
over that
])ortion
of the llumacao
district lying
b(4we(Mi
the
(iuaya]u\-
ami llumacao drainage
areas and that
])ortion
occupi(Ml
by the (\iguas
lowland,
the rest of the region
reaching a matnre
stagx^
of
(U'osion.
A
slight
sul)]nergt^iU'e
followed
the
second
])artial
pene})lanation,
which
affected
the western
portion
of
the
island
in
])articnhir,
for along
th(^
northern and
southern
coasts
extensive marine
deposits
of
middle and
u])per Oligocene
(rTul)l)ard, ID^ih
p.
0(5; Loheck,
lir^l\
p.
:)1.S)
age rests
upon
an
erosion
surface
that
is
contem])orane(nrs
with
the lower
])ene-
])lain
of
the llumacao
district.
Then^
is no
direct
(Evidence,
however,
that
any
of
the
llumacao
district
itself
^s-as
s\d)nHn'ge(l.
as
nowhere
within
its borders are there any Tertiary sedinu^nts.
The submergence
of
a
])ortion
of
the
northern
a))d
southern
coastal
margins during Oligocene
tinu^ was followed by another
niplift whidu
in
th(^
cas(^ of the Huma<?a() area,
n^sulted
in
a rejuvenation
of the streams
and a marked increase
in their eroding pow(MX
Somewhat later, a drown-
ing of the entire coast line occurred whidi
r(\sult(Ml
in
tho
formation of
irumenms
large
estuaries
that
are now
fillfMl
with
aUuvial
(\i^\)V\^ and
constitute the playa areas of the
south(^ast(U'n
coast.
Lobeck
(102-?,
])|).
346-:i48)
attributes the drowning of the coast
to a
rise of s^a-Ievel, due
to the nudting of
i]w
ice stored
up
in
the great continental
glacitvrs
of
Nortli America and Europe at the dose
(vf the
rieistocen<^
ej)0( h.
184
."^CIEXTIFIC SmVEY OF PORTO NICO
VnKvut
cluiiiges
ill
tlio Hniuacao
(listrict liave consisted
in
the deposi-
tioii
of
alluviin])
along stream
\alleys and
in drowned
hays and
as
al-
luvial
fans along
the
south
eoast.
Evidenee
for
a
loeal
eniergenc-c^
of
])er]ia|)s 25
to W
feet ah>ng ])ortions of the eoast also
exist
in
the form
of wave
cut
terraees,
raised
])eaehes,
and
eliffs
ent
into
I'lHHMit
])laYa
de|)0sits containing marine fossils.
K(^()X()MJ(^
(;F.()LO(iY
A]A(JX!7riTK
I) K POSITS
Magnetite
deposits
that
liave
attracted
some
attcMdion
as
possihie
future
sources
of
iron
ore occur
at
a
nund)er
of
localities
in
ihe ohhu"
series of
tlie llumacao
district.
The ])rincipal
{)n(\^, which
have ah-eady
heen descrihed hy the writer in
a })revious |)aper
(Fettke,
1i>*^4)
oeeur
in
association
with
the
l)elt
of
('(dlores
limestone,
trending approximately
S.
5:]°
E. wlneli
l)egins It^
nndes
east of J uncos and can
he traced
for
a distanee of al)()ut
(>
nn'les toward the coast.
Four outcrops of magnetit*^
were
ot)ser\e(l
along
it.
whicli
are
designated
as
Xos.
1,
2,
i),
and
I,
respectively,
starting
at
tlie
northwestern
end
ou
tli(*
ge()l()gi('al
map
accompanying
tliis
report.
Otliers
are
j)rol)ah]Y
present
hut
(^sca])e(l
notice on account of t!ie thick mantle of resi<lual soil wide!)
in most ])laces
conceals
the
l)ed
I'ock
fornuitions.
One
deposit,
D/lO
ndle
south
of
Torres,
designated
as
Xo.
o,
lies
outside
the
luain
helt,
heing
al)out
T/10 mile
northeast
of
it.
Other
dcjxisits
of
a
sindlar character oeeur
in the older series,
o or
-1
nules west of d uncos, a short distance south of
the duncos-Oaguas road, and
in
the
Inlls
north
of Arrovo.
The
writer
did
not
have
an
o{)|):)rtunity
to examiiu'
these
personally,
hut was
in-
formed
\)y Professor dames
F. Kemp that
tliey nrv v(uy similar to those
lying east of J uncos.
Tlie outcrop at locality \o.
1
east of d uncos makes the most prondnent
showing and this de])osit
is prohahly
the
largest of those (wannned.
hi-
asnuu-h
a>^ there are
jio
fuels availahle
in
I'orto
KNco
for
the
estahlish-
nient of a local iron industry, the ores, if ex{)loited, will ha\'e to !)e shippcMl.
The two nearest harhors at which
shi{)])ing facilities are at
pi'es(Mit avail-
ahle
are San duan and
Fajardo
Playa.
Of
these, San duan
is
hv
far
the
l)ettei'.
By way of the (Uiraho and
Iiio Orande (h
Eoiza
vallev, the
d uncos
dej)osit
is
.'U
miles
distant from
San
duan.
The
electric and
narrow gange steam
railroad from San
ehian
to Oaguas covers
tlie
dis-
tance
to the mouth
of the Ouraho
lii\er.
From
there
it
is
10
ndles
ui)
tl)e ijuraho
vallev
to the magnetite
deposit.
Along the greater
portion
of the latter distance
a
light Jiarrow gauge
track
has alreadv
heen
eon-
F/JTTKfJ, (UJOIJXjy OF THN
III MACAO DISTRK'T
IS5
sti'iu-ted
for
the
transportation
of
siio-ar
cane from
the
fields
to
the
central
'%) uncos."
Bv way
of the
Jieadvvaters
of the
(xural)O
Kiver,
across the divide
to
the
Rio
Blanco
valley, down
it
to Xagiiabo, and thence
[)arallel
to
the
coast to Fajardo Playa, the distance
is al)out 27 miles.
Ensenada Honda,
p,,;
4s.
h'clat'ioi)
of
wdffnctifc outcrop
\o.
1
to ^urroundinn
foniinf iou.^
Houth of
I^Vjar(h) Ilarhor,
is
T ndles nearer, hut no
shij)))ino-
facilities are
at })res(Mit availahle.
Along all hut the iirst
1) miles of this distance light
narrow gange
tracks
are
already
in
existence
h)r
tlu^
liauling
of
cane.
T\w
onlv
stec})
grades
along
this
route
occur
after
leaning
the
lu^ad-
waters of the Guraho Biver and going down Pena ro])re
(h-eek,
a
trif)u-
tarv of the Bio
Illanco.
Along this
tlie hanl would he down grade.
TUv mniiuvtilv
tli-|H,<ir
at
lu,-alily
.Xo.
1
hv< 1%
m\\v< east
«.r .hitu-os
uuii lUt vvv<l nf a
ri.luv,
s<.iilli
..f
ilx'
<;iiralM>
vaJlev. that
rise- appruxi^
rtivly :JM
feel
above the
h.wlaiuis
h>
the
iioiih ami
>ova\\.
'Fipir..
4H
.nsistv
uf
ii
>k.-.u.h
inai),
lja>«Hl
ii|Hai
lielil
iiote^. whirh
<!i.nvH
tlio
reliV
niislii],
<.r the miicrop
u,
the
siiri-<niji<liii,a
I'orjiiiil ions
: while
(iuiira
4«>
a
plinto^a-aph
t.f the rid-e as
it
appeiiiv from
tlio
s.mili
lonkino^
north.
The oiitfrnp has a strike of \.
(id
'
\\\ and apparent
I v
<lips
li'^- toward
w
iiurfheasi.
jr
eaii
Ite
irai-etl
h.r
a
(lista.iiee
«.l' 'HMM
fee!
ahaiiu-
tlio
vsi
of
the
rid.uv.
hut
the
hesi
(h-veh)fimeiit
of
JiUl.mielih'
ueeiirs
at
the
.rthwesi
taid. where hiruv huuhlers and, pnairincnt !ed-es
nl' nearlv pure
a-iiePile
juake
a
v.-rv
'eoiKi.ieiious
showiii.i!-.
.\hieh
..f
the
niaguetiU'
laMii
!?
'
i.
appeariii.ii'
in ihe uuter<.p ar the northwe-si end
is
line .yi-ained ami
neMrly
pure.
lt]w>s('s<vi^ a port.us struetiire, thie iindonhtiMlly lo the
h;-rtehiiig- out
id: siJicales and perhaps snndl aintnmls id snlphides
lyv
wertiherin.i--.
(h-hv^
hedral
(awstals
of
uia,,a-rict:ite
ai'e
snnieiiine-
seen
liiun.!:^
(haisy
ea,vilies.
.Part
of
itif
nia,2aK'tite
is
suiliiaently
niaaaietie
to
piek
up
siriall
tarks.
Oeeasionallv
a
line'
ji-rairied
intcr^u'rowlh ot
nu^.y-netite and
speeulai'
.heina.-
tite
oceui's.
Qualitative
aiuilyses
slww
only
traces
of
titaninni.
Jn
several ].lares, a
lirile nialaehite was untieed.
This
is presunujhly derived
IroDi
a
sulpiudJe,
sneli
as eludeopyrde. and
indieales
thai
in
dei>th
snurll
ananijit.s
of snlphides are
proi.alily
|n-eseui
in
associatitai
with
ihe
nu-i.y-
netda-.
thirnet (teenrs with the
uia<:iH'tite at the ivesl end
oC ihe
de|)osit.
ruder
the
niierosenpe.
it
exldhd,s
pruu.uuiiee«l
opiieal
araanalies
and
<:ranp1e-x
twuriin.ii-. A
little
lihrous auiphihole has
hecni
huine.l
frojn
it by
Ft:'l''l KL\
<!K<)IJ'H!y OF THH HI'Mli'Mt
IH STRICT
is;
iiltcnvlioii.
Ai
lni.
vn<i
i-ml.
the
lua-nriit,.
.H-.-urs
with
Juuch
Inr^^t-r
aoioiifiiM of
-^ilu^Hi.-.
A
ihiii
MH-liun
of
a
s|.rriuN/n
.<huv;> epidorc
t..
I..-
the piviUnmw.mi
miin'ral
willi
which
ihc
nin^mici itc
is
associaietL
i.m
s^ihmII
pati-lu's
i,f y'linicf nrv
rilsf.
|ir.'.-wit.
M.^iirc
:.(i
is
si
phnioiiiit-rugriVph
that of
iiui-iHi!t"
an.
I
t'pidnk'.
>;uv.>(>.h.il
hv
a
(.f
hf
ro!IlMH'>:
rnorphir r.uk
ivhirh
i>
Ih-Hi'v.-I
h.
h.ave
limc*ti>nc.
pi-riiap^
ii
1ufrai-cou>
jihan'.
X
out along thv hWhuU'.
A
I
niii. phicMihoi
iho strike;
IK
N.
;ir^
W.
aiHi thr
.lip
:iN-^
X.
K.
I.arc.'
vug.* ocrur
it
A
iin(T..H-..pii-
cvaiuiiialicu
of
ih.-
au.l
-arn..t. the
s.niriH-t ha
pill
ftM't
brvr.llil
Ih
mcteiuoi'pr.csc!
'.<'<-^ m
!'"
rcvoalH that
it
>-un<l'^\^ jargclv of pvr
.
iIm:-
pvri.K^'iie.
Calcit.-.
i-puluu.,
quart:-.,
am]
tah-
ai
,,..
;,uinniiT<.
(Iiloritc
ha>
P..v,.lupM-.I
along unnuU-
frai
developed
prc.«.irt
i
iss
srft:\'nFH'
ss(i,'ri-:y of
I'oirro
uir
tuivs.
AiH.tlier hiHH-iim.)i
i;ik,.ii
from
tin- walls of
uiii.
..f thv vui^h mnmnU
hiruvlv
ul*
--aniet anJ ampliiholo with
soim.
U\\v.
Thv
-;:.ri!=.f
Hiipiirenlly
luniuMl
first. Ihcn
tiu' Hinpiiihul,., aiul
liiiiilly the Inlr,
llnnihleiule
ainl
pyrr.wuv
.-.rhi.-^rs
iiiHloul.tc.lly
denv,..!
irnii!
luidcKiKi
lu!l>,
t.vi'riic and
iiiiil(»rlic the nu'lamorpluKsed
linicsloiu'.
A
tiiin
scrtioji
of
a
,-4jH-riinen
uhtaiiied
from
aln-v*'
tii<>
Jiiupstoin^
|h.^.m'>^-=i'-
n
iliHliucidy
fnliatcrl
>tructuiv.
Ir
i<
iinde up
cidctiy
of
pla-KHda.^i-.
.|narf7.,
oiilai-
cla-u, and
a
dci'j.
.uavcu
hoi uiilcudc.
A
little pymxv^iv
ir~
al>u
|. resold.
I
I
I
LJ^_J
Fill'
(.iitliiu's of
a
few ono-inal
lVld..par
.'ryslals.
lanv
ri,iH|d,.-i('l v
altered
<i
kaolin,
<eriMtv. and
..pideie. ran
still
he
]T.e.,miixe.L
Mapieiiie and
itaiiile are piv^enl
as miner euiisritut-nls.
.\ia)|,her >pinanHni taken frem
I
(MHid seme dislanee
h..|(.u
th..
liinestene
alse
in^<^v^^v.
a
sehi>tiw
a|.-
H-aranee.
It
is
eompused
ef
an
interhjeking^
nu.saie
of
pale
lim^n
»yruxeiu-,
pda.--iuelar^e. enlioelase. and a
lillle dark
,Haver=
hiH-nhlende.
A
.eu
larse
ervstals
uf
pvri.xene
full
ef
inrhisien^.
inrludijiu^
smaller
FfJTTK/'], aiJ()L()(JV OF THIJ HI MACAO DISTRICT
IH!>
pyroxene
<i:rains,
are
])rese]it.
^la^nietite
oeeiirs
in
minor
amounts.
Hpidote
lias
(levelo])e(l
alono- minute
fraetures.
At the west end, an angite andesite oecnrs as the
Footwall of Ww mag--
netite
deposit.
It
possesses
a
felsitie
texture
witli
a tendency
for
the
many fehlspar
laths of the o'roumhiiass
to
he
in
parallel
ali^iinnent,
in-
dicating-
a
tlowag-e
structure.
A
few
pheuocrysts
of
])lagi()clase
of an
amlesine
or
acid
lahradorite
variety
and
a
little
augile
are
])resent.
The
matrix
consists
chieHy
of
plagioclase.
Titain'te
is
an
accessory
jnineral.
Pale ^vreen
horn])len(le aggregates occur ah)ng snudl
fractures,
as well as
in places once occupied by augite |)hen()crysts.
The ])lagioclase
is clon(hMl
\)y alteration products.
In
one
])lace
north
of
the
deposit,
an
outcrop
ot
nu'tam()rj)hose(l
andesite
tuff was
ol)served.
It
]){)ssesses
a
hue
textui'e and
(vxliihits
a
slight tendem-y towards a
scliistose structure,
(irains of
augite,
plagio-
(dase, and
orthoclase
u])
to
.4
millimeters
in
length,
ai'c
pi'esent.
The
matrix
consists chiefly of an
alloti'ioniorphic
int(u*growth
of
(puirtz and
])ale grecui hornhlende.
A
little chlorite has develoj)ed from hoi'nhlende.
Beyond the andesite
tuff toward the northwest,
therc^
is an occurrence
of horid)lende andesite p{)r])hyry.
Augite pheuocrysts with
reaction rims
of hornhhuide
ai'e prominent.
The
nuitrix
is made up
i)i"(M]oniimint
(d'
a
hrownish-green hornhlende with
only minor amounls
of
]>lagio(dase.
It
is coarse
in
tc>xture.
:\
little magnetite and
apatite^ are
pi'esent.
The
J uncos
deposit
lies
af)out
2J){)i)
feet
north
of
the San
Loi'cnzo
(juartz diorite hatholith.
An
olf-shoot of the
latter, which
is
pai't of the
Lu(piillo stock, crops out 500
t'eei
north
of ihe magni^ite occuri'cnce.
The magnetite
<le])r)sit
at
locality
No.
2
is
situated
ahout
'A'-y\
mile<
southeast
of
Xo.
1
on
the
south
side
of
a
ridge
ahout
1<)0
feet
helow
its
crest and
'M\^)
U\A above the
^•alley
at
its
base.
The oulcro])
strikes
X. 50° W
. and can be traced
U)V a distance of X\T)
feet.
Tlie dip appears
to be 72° toward the northeast.
]\Iagnetite
is
the
chief
ore
mineral
pr(\^ent
but
sonu'
fine
grained
specular
liematite
also
occurs,
(hirnet
is
thc^
j)redominating
gangue
mineral
ai)pearing
in
the
outcr(»j).
Drusy
cavities
lined
with
tra])ezo-
hedr(ms of garnet and sometimes small octahedrons
of" m;ignetite as w(dl
as crystals of (piariz are of connuon
occurrence.
\'einlets of magnetite
and
(piartz are seen cutting massive garnet
rock.
The garnet
])ossesses
a
reddish-brown
color.
A
(puilitative
analysis
imT ica tes
that
it
is
the
andradite
vai'ietv
silica,
iron
oxide,
and
linu*
being
the
chitd'
con-
stituents.
.Vlumina occurs
in only snuill amounts and magnesia
is al)sent.
A
green
brecciat(Hl
rock
forms
the
north
or hanging
wall
o!'
the de-
i)()sit.
Tlie dark green
|)ortion
is
intersected
in
all directions by streaks
|()()
.^CIEXTIFJC i^lRYEY OF PORTO RICO
of
light
i-ulortHl
niaterial.
(Jiuler
the
microscope,
the dark
greeii
por-
tions
are
foiiiul
to
consist
chiefly
of
pyroxene
associated
with
small
aiuouiits
of
garnet.
The
light
portions
represent
an
allotrioniorphic
intergrowth of plagioclase, orthoclase, quartz, and some pyroxene,
ft
is
thought
tliat
tliis rock represents a metamorphosed andesite
tuff,
l>i'<>h-
al)ly a calcareous one.
North of the tut! along the crest of the ridge, an augite andesite crops
out.
This
iuis a
felsitic fexture.
Under
tlie microscope, the plagioclase
laths
of
the groundmass
are
seen
to
he
in more or
less
parallel
align-
ment, indicating a flowage structure.
A few hirge phenocrysts of augite
can
still
he
recognized.
Dthers
have
been
altered
into
aggregates
of
cpi<h)te or hrowuisli-green
hiotite and secondary
(puirtz.
The
Jiuiti'ix
is
(•oni])osed of plagioclase
lafhs,
hiotite. and augite.
The above magnetite deposit
is located al)out
V-^ mile noi'theast of
tlu^
San Lorenzo batliolitfi.
Idle
outcro]) of the maganetite deposit
at
hjcality
iSo.
v), which
is
t/o
mile souttu'ast
c>f Xo.
2, while
it does not make a xery conspicuous show-
ing at
the
siirface,
is of considerable
interest because the relationship of
ihe magnetite and
contact metamorphic minerals
associated
with
it
to
the Coheres limestone
is brought
out more
clearly
tlian
at any
of
the
other localities.
The magnetite associated with the metamorphosed lime-
stone occurs very near the top of a nortli west-southeast ridge.
Numercms
outcroj>s of nearly jiure crystalline light gray and white limestone api)ear
on the southwest
iiank
of
this
ridge.
The
strike of
the formations
is
X. 54
""
\y. and
tlie dip
is
:)2° N. E.
It
is estiuiated that the limestone
has
a
thickness
of
at
least 250
feet.
A small qnarry has been opened
in
it at one place from which limestone has been taken for burning into
lime for local use.
Qualitative analyses of
tlie limestone indicate that
it
sfiould yield a good grade of lime.
A thin
section
of the crystalline gray limestone below
tlie magnetite,
rcNcals
an
interlocking
mosaic
of
calcite
crystals
of medium
texture
with only
occasional minute grains
of iron oxide amongst them and
a
xei'Y
little
clilorite.
A specimen
of
the metamorphosed
limestone from
the hanging
wall
of
the
deposit
is composed
of
garnet^,
pyi'oxene, and
calcite—garnet
being the most abundant.
Garnet seems
to have
devel-
op(?d somewhat later than pyroxene, and some of
tfie calcite in the form
of
little veinlets cuts
all three.
Northwest of the deposit proper, promi-
luMit
ledges
crop
out
that
range
in
mineral
composition
all
the way
from crystalline
calcite associated with only small amounts
of
silicates,
to those in
whicli
silicates are the predominant constituents present.
A
thin section of the least altered rock consists mostly of calcite and a little
FJJTTKE, (iEOLOay OF THl] lUMAC \0 Dh^TRK'T
\\)\
garnet
intersected
by
fractures
along
which
quartz,
specular
hematite,
and possibly magnetite have been introdnced, replacing
tlie calcite.
This
was followed by the devek)pment of chlorite along fractures and between
c-alcite
grains^
replacing
the
latter
in
part.
The
final
stage
was
the
depositicm of later calcite in the forju of minute veinlets cuttijig the other
minerals.
The limestone
is overlain by an andesite at locality Xo.
;] and under-
lain by a metamorphosed and much altered andesite tnlf.
The magnetite
(h'])osit occurs in the upper portion of the limestone just below
its con-
tact with the overlying andesite.
Tlie San J.orenzo batholith
lies
al)out
V;^ mile to the southwest.
The outcrop of the magnetite' deposit at locality Xo. 4 occupies neai'ly
the entire
crest of a
hill about V/U ndles southeast of Xo,
-"h
It
lias a
length of nearly 200 feet and a maximum wi(hh of 80 feet at the western
en(L
At
this
end
scmie
coarse
inicaceous
hematite,
associated
with
(piartz,
as
well
as massive magnetite,
are
present.
The
outcrop^
wliile
not very large in area, makes a V(^ry prominent showing.
(Ireat boulder^
of massive magnetite derived from
it are strewn along the hillside to tho
south and have accumulated in
tlie little \alley at the
}>ase.
A feldspathic andesite comprises the
iiajiging wall of
tlic
de|K)sit.
In
fhiji
section,
it
is seen
to possess a
felsitic texture and
distinct
fiowag<'
structure, brought out by the parallel alignment of the iplagioclase
latlis,
A few phenocrysts
of plagioclase and
a
light green
]iornl)lcnde
(hn'ived
possibly from augite are present.
Occasional aggregates
of epidote also
occur.
The matrix consists predominatingly of ])lagioclas(^
laths
in more
or
less
parallel
alignment.
Skeleton
crystals
of
iron
oxide,
possibly
magnetite,
are prominent.
A
little hornblende
is
also
pr(\<ent.
Three
hundred feet south of the deposit there
is
a small outcrop
of
a
sericite
phyllite, probably representing
a
metamor])]iose([
volcanic
ash
or
shale,
which has a strike of X. 59" AV. and a dip of 41° X. E.
S(4'icit(\ quartz,
and
iron
oxides are
its
chief constituents.
One
thcmsajid
five hnndrcnl
feet X. 73° W. from the deposit prominent ontcrops of gray
crystalline
limestone
and
associated
garnet
rock
were
o[)served.
I'he
main
San
Lorenzo batholith
lies % mile
to the
soutliwest,
wliile
a
small
oiTshoot
fr(mi it crops out only Va i^i^^ away.
The magnetite
deposit
at
locality Xo.
5, 9,10 mile
south
of Torres
and about the same distance north of
• Xo.
4,
do(\< not belong to the same
belt
as the four deposits already
descri))ed.
The cmtcrop,
whi(4i
is not
very conspicuous, being traceable only for about
50
ivvt, occurs on
the
]K)rth side of a ridge just below
its crest,
wliicli
rises 410
feet abo\e
tlie
PJ2
SCIKXT/FIC Sfin'E)
OF
l'(pRTi) RfCO
(iiirabo
vjillt'v,
FiumH'
:^2
is
a
|ihuto<;Trtph
uf
tlu'
riil,u<'
rt«
H,
Mp|H'rtrs
fruin the north.
Oil
tin.
iicrtlH-ast
si<li' of
tlu'
rid.i.T. 220 W-A
In'hnv
tiu>
liia.uiiotitc
i)rus^
|iiM-t.
u!
(.'h'\atii«ii.
<.iin-rn|)> of
!iini'>toiH> aro cxposiMl. one
Icdgv
luiving
a
thickiies>
ul"
1(1
ftH-r.
Oil
acroiiiir
of
the mussivp
t-luiractor
ol'
the
rock,
no
sf.rik*^ am! dip lueasuivnu'iit^ on
tlic
liiiH':^toiu«
ilstdf wvrv pos^hh'.
It:
is
ovt-rhiiiu
liowev»«r.
I»y
an
nuu'ilc
aiidositx',
^howiiij^-
tlowagf
si nu-iuri'
imdor the
!rii<i'os<-op(\ ouf(Top|tiiiu
UmIo-cs of whirh occur
in such
relalioii
t<» the limestone as
to
load one to conclude that
the
r-trikc of the i-ontad
of
th(> two
is
X.
is"
W. and
the dip
:>'>''
S. W.
This makes
it appear
thai
a svnelinc rcrur^ hefwcen
tlic depo^it^ of
Xos.
I and
5 and
that the
limestone
at
lr(^alilv
Xo.
o may
In-
the
sanu'
hed
that
the
other
four
diiinsits areas^^ociat-'d
with.
W^^-o.*...
;*c^i
A
jiiirru-copic
.'xainination
of
the
limestmic
reveal>
a
rather
Hne^^
u-rained
rock composed
chiefly of an
iiiterlockiii.u-
jnnsiiie
ol' calcite
erys^
tals
with
occasional
-rains of quartz,
aviu'ite.
aJid
pla-iochise
distrihulet!
mnon-sf
them.
The
latter
undcnl.tedlv
represent
tulTacemis
.materia!
deposited contemporaneously with the ealean-ous constil ueiits.
On.' hun-
tired and seventv
feet >outheast of
ilie lunestone outcrops
alon.u' the
Inlh^
side
a
proniment.
Icd.u^e
of
inunwi
rock,
dev.dopi'd
from
the
limest.aie.
was
nol u-ed.
The
^irnet,
which
is
appareiilly
an
antlradttc
variety.
sho\v< hirefrineence and
is
np|icall\-
nepitive.
It
was
at
(irst nustakeu
for \a'su\danite
in
thin section, fmt posse»cs
a
liigher
ind(>.\
of
rtdd-action
than the hitler
inin<n'al.
FETTKE, (UJOLOaV OF THE HI
MA(\\(> DISTRICT
YX\
(i()in<>-
in the (lirectioii of
dij), augite aiidesite and
latite occur between
the
liniestoiie and magnetite
(mt<TO})s over
a
belt about
'^.')0
feet
wi(U\
Whether
tliese are extrusive or
intruf^ivf^ sheets could
not he deterniined
from their
field
relations.
The andesite
wln'ch immediately
oviudies
tlie
limestone,
in
thin
scH'tioii, sbows
a
parallel
alignmeiit
of
its
plagioclase
latbs,
indieatin<^ a flowa<>e
structure.
The
wall
rocks
of the maixnetite
d(^|)osit
itself are not exposed.
A
latite
tuif ovcudies
tlie
nui.u'uetite, but
it was not o})served
iu
direct contact with
it.
Tins
in turn
is succeeded
by dacites and
andesites toward
the soutbwest.
Tbe magnetite
deposit
lies
IV:^ miles
n(U'tlieast
of
the San
Lorenzo
batbolitb
contact and
l-/;;
miles southeast of the
lAKjuillo stock.
The
maguetite
de|)osits
described
are
cl(>arly
of
tlie
type
ordinarily
termed contact metamor])hic deposits.
In
eacli case tbey
[)oss(\>^s mineral
associations and
oc^-ur
in
such
relation
to
the
sun-ouinling
rocks
as
to
lead one to conclude that they have been formed
by the
re])lacement
at
high
tem])eratures and
])ressures
of
either
a
nearly
pui'e
limestone,
a
tulfaceous
lim(\stone,
or
a
calcareous
tuff.
A
study
of
ti\(^ geology
oi'
southeastern
]\u"to
Hico
leads
one
to
tbe
conclusion
tbat
tlu^ magma,
wbich gave
rise
to the
Saj]
I^orenzo
(puirtz
diorit<^
batbolitb,
as
well
as
the Lufiuillo and other smaller satellitic stocks, and from
wliicli
tbe later
intrusions
that
surr(uind,
and
in
soim^
instances
penetrat(>
tlie
(piartz
diorite,
were
derived
l)y magmatic
differentiation,
was
also
responsible
for tbe (mianations tbat formed
the magnetite deposits
in
tbe
luiu'e
cal-
careous portions of the older series of eastern
Porto
liico.
Tiic pi'oximity
(d' the San Ijorenzo batbolitb and tbe
liUipiillo
stock,
as
wtdl
as
sniallci*
olfsboots,
to
the
magnetite
deposits,
indicates
tbat
tbe
<piartz
diorite
undoubtedly underlies tbem at no very great
<leptb.
The metamorpbism
])roduced
by
the
intrusion
of
tbe
(piai'tz
dioriti^
into the older series was of two kinds.
In one case
it
involved simply
a
recrystallization of constituents already
pres(Mit
in tbe original
rock with
very
little change
in
cbemical
comj)osition.
Tbe
Ik^iI
of
tbe
intrusi\«'
and
probablv
tbe escaping water
vap(n%
as
well
as tbe
lu'es^ure
exerted
by
tfie invading mass, were largely responsible
for
tlu^se cbanges.
Horn-
blende s(^hists were in uuiny instances developed from andesite tulfs along
the contact, as has already
Ixn^n
pointed
out.
In
a tew
])laces, where tbe
intrusions came
in
contact
with
sbales
and
sandstones,
pbvllites
and
(piartzites were
formed.
The conversion
of an
ordinary
limestone
into
a
crystalline
one, such
as uow
occurs
in
tbe
vicinity
of
tbe
magnetite
deposits, with
the obliteration of
all
traces of organic remains, can
like-
wise be attribute<l to the sanu^ cause.
In tbe case of tbe formation
of the magnetito deposit^ and
tlx'ir
ass(.-
!1M
••IKSl'lFlv sfllVEY
<»F
I'OirFfP Rid)
r-iatid
,^ilif.-it..>.
nil
rhe
utlipr
hnii.l,
.'inaiirtt iojis hum
the
inii.i;-i!ui
itself
ii!frO(lu(-'0(!
Inruv annmiits
.if
iniii.Tiil
matter,
eoiisistiu-' vhwih uf
silica
and
iron
o.\i<Ie<. viiirh
n-«ft<'<l
with
hih!
tu a
.uT.-rti
oxteiit
ivi»lfu;i'.|
the
ongirinl
crtlrii,-
oi;
th,.
linn.stnnr-
and
vnlvnmms
tulT
with
whiVh
th.-
.Icposifs are assoi-iated.
The
jiiiiir.falizetl
eiiumariniis
rr..tn
rhi-
(.lulino-
.|iiiiriz
dioritc
ina.i(iiiu
vn.^e
aioji.u-
;frarfiire.<
thruiiul,
rhr
Hirrnnndiiig-
-rf.<'k>
in
rho
ininKMliafe
vieinity
r.f
tdie
ininisivc
imlil
thiw
eariie m
idrti;cs
wh./re
llie
rra.1iin><
inrcr.^eried
the
m-arhy
hiiiestuKes Hud
(^dc^aroous
tulfs.
The
htun:
r.-
aeted
wifli
th."
Ierru-siliee..ii,'< cmannt iuii^ miwh
im.rp
rea<lilv
fiiaii
the
and(>>ire>aiid nrdiiuirv
tiul'^ with whirh thev are iiitaa-hefhled and
re|d,a«-.-
th-
IwlT
*.ki-^-
"
"'
3%g
iruii ore deposits ihiavrnre. the bells alonjr whieh
the ealeareuiis
iiiuld<aas
Hf the older series erop
,,ut,
ill
the viididtv of the San
Lorei.izn,hatJi(dit,h
and
its
suellitie
stocks.
ar.>
tlie ones
whii-li
>!)uuJd
he
pro>|)eeletl
most
thorough ly.
Iloek syilal)Ie
iV.r
roai
MJ'TAI.
eojisirnctinii
js alrundanl'ly
distrihiits'd
ilirtniuii-
out
the
Ilhnnaeaii
district.
Ainonext
l!a>
various
ty|H-.s
that, have
be.'ti
n<v<l
lor this
|Hi]'|i"se iire
]n<'liid,etl
fidsite.
rhytilite,
lalite-, daeite, ainlesit.-.
qnartz
diorite.
andesitht
InlTs and
hna-eia.-. and
raUearenus
ivdl's.
The
andcsite.^ and
latiles are prol)aldv
Ihe
la-st
for
nuiertdani
road
eonsirn<-
tioii.
Fi-uiv od shows a view
id' one of the ]ar-vr rpiarrirNs located alon^^
the
Hiiliraiw
road
.soiitliwesr
of
('ayaia-.
a short distanee nn
tlie Qyehra^
dillas vailey.
Idle
roek obtained
Itore
is
a hornblende
bitiie.
FF/rTKii aijouxn
of the humacao district
id:,
LIMKSTOXi:
The Collores limestone was
tlie only one encounterei] within
the
])or-
(lers of tlie Hnniaeao district that
is of siiffieient puritv to
l)e snitahle for
])iirnin(2: purposes.
The hest (levelojnnent was ol)serve(l
in tho vicinity of
the harrio of CoJlores de Piedras, where
it has a strike of X.
^)A° W. and
dips
32°
toward
tlie
northeast.
Prominent
ledo-es
of
^ray
and
wlute
crystalline
limestone,
in
some
instances 40
feet
thick,
croj)
out.
The
total thickness
is estimated to he 250
feet.
Qualitative analyses indicate
a very pure limestone.
There
is very little residue after treatment with
cold dilute hydrocliloric acid, and only slight amounts of
ii'on oxides and
ahimina and
a
trace
of magnesium
carhonate
are
present.
A
small
(piarry has heen opened in
it at one
]:)lace from which limestone ha^ heen
taken for huivning into lime for local use.
BRICK CLAY
While the residual
soils
of the u])land
ar(>a
of
tlie llumacan
district
are predominatingly of a plastic ferruginous clayey nature, they are not
snitahle for making a good grade of huilding hrick.
The writer did not
come across a single locality in the district Avhere hricks were heing made
from them.
KAOLIN
At several places along the military road southwest of Caguas and west
of K. 45, rhyolite ])orphyry dikes are exposed that have heen almost C(un-
pletely altered
to kaolin.
In some instances these are almost pure white
in
color,
Avhile
in
other cases they are hadly stained hy
limonite.
The
<puirtz
phenocrysts
of
the
original
rlvvolite
])or|4iyry
ai'c
still
])laiiily
visible in the white kaolin.
These could prohal)ly he r.>mo\'ed by
(^arefiil
washing.
There
is a possibility that commercial
(le])osits of kaolin may
be present in this locality.
THERMAL
S PRIMES
One thermal spring, the Yirella, located out on \\w ])laya northwest of
Arroyo,
lies within
tlie borders of the Humacao
district.
ITodge
(102t).
pj). 225-22())
quotes the following analysis made
l)y
^\v. Louis irernan-
dez,
of
the
Porto
Pican
Service
of
Sanitation,
of
the
waters
of
this
spring:
Ich;
SriEXTIFW survey of PORTO RICO
Quantitative analysis
Amon lit of water
Tarts uer
used.
("'ul)ic
million.
centimeters.
Nitrates
02
100
Fi'ee
aniinoiiia
04H2
....
Albuminoid
ammonia
04S
....
Total
annnonia
01)12
KKK)
Potassium
(W,)!
5(X)
Sodium
819
500
Litliium
00287
lO(KK)
Sulphates
4598
1000
.Alau^nesium
(K>12
1000
Calcium
1
. 0877
KKK)
Olilorine
l.:]5790
KM)
Silicon
dioxide
:i078
KKK)
(Vn-bonates
as
(^O,
1.00577
KK)
Feiric
oxide
00402
5.94702
1' »rts per
million.
Sodium
nitrate
i \7H9
Ammonium
chloride
l.*)74
Lithium
chloride
tM).*>4o
Potassium
chloride
1409
Sodium
chloride
2.1991
Ma.irnesiinn
suli)hate
(K)4
< 'alcium
sulphate
1
. K)88
Feri'ic
cai'bonate
00759
Silicon
dioxide
*U)78
Calciiuji
carbonate
1 .7721
5.82K)2
Total
solids
5.8242
Q ua litativc ana his is
I'romine
neij^ative
Iodine
nej^ative
HydrojLren
sulphide
ne,ij:ative
Carbonic anhydride
(free)
nejj^atiA^e
Free
suli)hiU'ous
anhydride
ne,u:ative
l*liosphoric
aidiydride
ne.irative
P»oric
acid
negative
Lithium
(speetroscopically )
1 positive
Specific
.gravity
1
. 0(K)
Tem])erature
.*>0° C.
Alkaline to litmus and phen(»li)hthalein.
FETTKIJ, GEOLOGY OF THE HUMACAO DISTRICT
197
Hodge
attributes
the high
chloride
content
of
the
water from
this
spring to wind-carried salt spray from the near-by coast and to the fact
that the
w^aters
issue through
recent coastal plain
deposits, which have
not as yet been completely freed of their connate sodium chloride.
Several
small
hath
houses were
at one
time
erected
at
the
Yirella
Spring, but these have now fallen
into a state of disre])air.
BIBLIOGRAPHY
Daly,
li. A.
1014.
Igneous roeks and their origin,
p.
90.
New York.
Berkey,
C.
p.
1915.
(^Jeological
reconnoissance
of Porto
Rico.
Ann.
N.
Y.
Acad.
Sci.,
XXVr, pp.
1-70.
Semmes,
I). R.
1919.
(Geology
of the San Juan
District,
Porto
Rico.
Scientific Survey
of I*orto Rico and the Virgin Islands,
I, part
1.
HouGE, E.
T.
1920.
Geology
of
the C'oamo-Guayama
District,
I^orto
Rico.
Scientific
Survey of Porto Rico and the Virgin Islands,
I, part
2.
I.OBECK, A. K.
1922.
The Physiography of I'orto Rico.
Scientific Survey of I*orto Rico
and
tlie Virgin Islands,
I, part
4.
Mitchell,
G.
J.
1922.
Geology
of the
I'once
District,
Porto
Rico.
Scientific
Survey
of
I*orto Rico and the Virgin Islands,
I, part
o.
HCCBARD,
B.
1923.
Geology
of
the
Dares
District,
I*orto
Rico.
Scieutiftc
Survey
of
l»orto Rico and
tlie Virgin Islands,
II, part
1.
Fp:ttke,
C. R.
1924.
Magnetite deposits of eastern Porto Rico.
Mining and Metallurgy.
No. 1294-M.
Feb., 1924.
m
I; ^mt M
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